structuring of surface for light management in monocrystalline si solar

STRUCTURING OF SURFACE FOR LIGHT MANAGEMENT IN

MONOCRYSTALLINE Si SOLAR CELLS

A THESIS SUBMITTED TO

THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES

OF

MIDDLE EAST TECHNICAL UNIVERSITY

BY

SEDAT BİLGEN

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR

THE DEGREE OF MASTER OF SCIENCE

IN

PHYSICS

AUGUST 2015

Approval of the Thesis:



submitted by SEDAT BİLGEN in partial fulfillment of the requirements for the

degree of Master of Science in Physics Department, Middle East Technical

University by,

Prof. Dr. Gülbin Dural Ünver

Dean, Graduate School of Natural and Applied Sciences

Prof. Dr. Mehmet Zeyrek

Head of Department, Physics

Prof. Dr. Raşit Turan

Supervisor, Physics Department, METU

Examining Committee Members:

Prof. Dr. Mehmet Parlak

Physics Department, METU

Prof. Dr. Raşit Turan


Prof. Dr. Nizami Hasanli


Assoc. Prof. Dr. Akın Bacıoğlu

Physics Engineering Department, HU

Assist. Prof. Dr. Selçuk Yerci

Micro and Nanotechnology Department, METU

Date:27.08.2015

iv

I hereby declare that all information in this document has been obtained and

presented in accordance with academic rules and ethical conduct. I also declare

that, as required by these rules and conduct, I have fully cited and referenced

all material and results that are not original to this work.

Name, Last Name: Sedat Bilgen

Signature :

v

ABSTRACT



Bilgen, Sedat

MS, Department of Physics

Supervisor: Prof. Dr. Raşit Turan

August 2015, 108 pages

Texturing of a silicon wafer is the first process of production of screen

printed solar cells to reduce the reflection losses by producing pyramids on the

surface of the silicon wafer. Being a cheap and time efficient process, texturing is

used in all industrial applications. For mono-crystalline silicon wafers, the process is

carried out by using an alkaline solution which consists of potassium hydroxide

(KOH), isopropyl alcohol (IPA) and de-ionized water (DI-water) which is heated to

75- 80oC, and wafers are put in it up to a certain time to get random pyramids on the

surface. These pyramids reduce the reflection and increase light trapping, and thus

increase the efficiency.

In this study, the magnetic agitation was used to optimize the process

parameters like, the concentrations of KOH and IPA, process temperature, and

process time. To minimize the process duration and process temperature, ultrasonic

agitation was used, and almost uniformly distributed small pyramids (3 µm in

average) were obtained. To observe the surface characteristics, reflection

vi

measurements and SEM images were taken. Then, the simulations of uniformly

textured wafers with textured one side and two sides were made to observe the

absorption ability with different thicknesses and different pyramid heights. Then,

solar cells were fabricated by using new process parameters and agitation.

To characterize the solar cells, reflection, external quantum efficiency (EQE),

current-voltage (I-V), and Suns-Voc measurements were carried out. It was observed

that cells based on wafers textured with new parameters and ultrasonic agitation

could be used to reach higher conversion efficiency values compared to reference

cells.

Key Words: Texturing, KOH- IPA Solution, Ultrasonic and Magnetic Agitation,

Pyramids

vii

ÖZ

TEK KRİSTAL YAPILI SİLİSYUM GÜNEŞ HÜCRELERİNDEKİ PUL

YÜZEYİNİN IŞIK YÖNETİMİ İÇİN ŞEKİLLENDİRİLMESİ

Bilgen, Sedat

Yüksek Lisans, Fizik Bölümü

Tez Yöneticisi : Prof. Dr. Raşit Turan

Ağustos 2015, 108 sayfa

Yüzey şekillendirilmesi silikon pul üzerinde piramitler oluşturarak yansıma

kayıplarını azaltan güneş pili üretim aşamalarının birincisidir. Ucuz ve zaman

tasarruflu olması, yüzey şekillendirilmesi işlemini tüm endüstride kullanılır

yapmıştır. Tek kristal yapılı silikon pullar için bu işlem potasyum hidroksit (KOH),

izopropil alkol (IPA) ve iyonize su alkalin karışımının 75- 80oC ısıtılması ve pulların

yüzeyinde rastgele piramit oluşturmak için belli bir süre bu karışıma konulması ile

gerçekleştirilir. Bu piramitler yansımayı azaltır ve ışık hapsetmeyi, bu da verimliliği

arttırır.

Bu çalışmada, manyetik karıştırıcı kullanılarak KOH ve IPA konsantrasyonu,

işlem ısısı ve işlem süresi gibi işlem parametreleri eniyileştirildi. Bundan sonra,

işlem süresi ve işlem sıcaklığını azaltmak için ultrasonik karıştırıcı kullanıldı ve

hemen hemen homojen bir biçimde küçük piramitler (ortalama 3 µm ) elde edildi.

Yüzey karakteristiğini incelemek için yansıma ölçümü ve SEM görüntüleri alındı.

viii

Sonra, tek taraflı ve çift taraflı düzenli dağılmış piramitlere sahip pulların

simülasyonu, bu pulların soğurma kabiliyeti farklı kalınlıklarda ve farklı piramit

boyutlarında gözlemlemek için yapıldı. Yeni işlem parametreleri ile güneş hücreleri

üretildi.

Bu güneş hücrelerini karakterize etmek için yansıma, EQE, akım voltaj,

Suns-Voc ölçümleri yapıldı. Yeni parametrelerle ve ultrasonik karıştırma ile

yüzeyleri şekillendirilen pullardan üretilen hücrelerin referans olarak üretilen

hücreden daha yüksek verime sahip olduğu gözlemlendi.

Anahtar Kelimeler: Yüzey şekillendirilmesi, KOH-IPA solüsyonu, Ultrasonik ve

Manyetik Karıştırma, Piramitler

ix

To My Dearest Family

x

ACKNOWLEDGEMENTS

I would like to thank Professor Raşit Turan for his support through all steps

of this thesis and giving me the opportunity of working with him and his laboratory.

Thanks to him for being a member of GÜNAM family.

I would also like to thank to Fırat Es and Emine Hande Çiftpınar for helping

me to work in the clean room without killing myself and writing this thesis. I would

also like to thank to Mete Günöven, Olgu Demircioğlu and GÜNAM family.

Finally, I would like to thank my family and my wife for their patience and

support along my studies.

xi

TABLE OF CONTENTS

ABSTRACT ............................................................................................................ v

ÖZ ......................................................................................................................... vii

ACKNOWLEDGEMENTS ..................................................................................... x

TABLE OF CONTENTS ........................................................................................ xi

LIST OF FIGURES .............................................................................................. xiv

LIST OF TABLES ..................................................................................................xx

CHAPTERS

1. INTRODUCTION ............................................................................................... 1

1.1 History of Photovoltaic Technology ............................................................... 3

1.2 Types of Solar Cells ....................................................................................... 5

1.2.1 c-Si Wafer Based Solar Cells ................................................................... 5

1.2.2 Thin Film Solar Cells .............................................................................. 5

1.2.3 Other Solar Cell Types ............................................................................ 6

1.3 Basic Concepts of Photovoltaic Science and Technology ............................... 7

1.3.1 Solar Irradiation ....................................................................................... 7

1.3.2 Basics of Solar Cell Operations ............................................................... 9

2. FUNDAMENTALS OF CRYSTALLINE SILICON SOLAR CELL

TECHNOLOGY .....................................................................................................17

2.1 Introduction ...................................................................................................17

2.2 Texture: Wet Chemical Texture .....................................................................18

2.3 Doping ..........................................................................................................18

2.3.1 P-n Junction............................................................................................20

xii

2.3.2 Modeling of Solid State Diffusion .......................................................... 22

2.4 Anti Reflecting Coating (ARC) ..................................................................... 24

2.5 Metallization Screen Printing ........................................................................ 26

2.6 Edge Isolation ............................................................................................... 30

3. FUNDAMENTALS OF CHEMICAL ETCHING PROCESS AND LIGHT

TRAPPING ............................................................................................................ 33

3.1 Introduction .................................................................................................. 33

3.2 Wet Chemical Etching with KOH/IPA Solution ............................................ 36

3.3 Surface Morphology and Light Trapping ...................................................... 38

4. EXPERIMENTAL PROCEDURES.................................................................... 43

4.1 PROCESS FLOW OF SOLAR CELL FABRICATION ................................ 43

4.1.1 Texturing of Silicon Wafer ..................................................................... 43

4.1.2 Doping ................................................................................................... 44

4.1.3 Anti-Reflecting Coating ......................................................................... 45

4.1.4 Metallization and Co-Firing ................................................................... 46

4.1.5 Edge Isolation ........................................................................................ 48

4.2 Average Pyramid Heights and Pyramid Density, Mass Loss .......................... 49

4.3 Characterization of Solar Cells ...................................................................... 52

4.3.1 Reflection Measurements ....................................................................... 52

4.3.2 I-V Measurements .................................................................................. 54

4.3.3 Electroluminescence Measurements ....................................................... 54

4.3.4 External Quantum Efficiency (EQE) Measurements ............................... 55

4.3.5 Suns-Voc Measurements ........................................................................ 56

5. RESULTS AND DISCUSSION ......................................................................... 57

5.1 Texturing Process with Magnetic Agitation ................................................... 57

xiii

5.1.1 Effect of Process Duration ......................................................................57

5.1.2 Effect of KOH Concentration .................................................................62

5.1.3 Effect of using a cap to Keep IPA Constant ............................................69

5.1.4 Effect of Magnetic Stirring Speed ...........................................................72

5.1.5 Effect of IPA Concentration ...................................................................74

5.2 Texturing Process with Ultrasonic Agitation .................................................77

5.3 Simulation of Light Trapping ........................................................................83

5.3.1 Simulation of SDE wafer ........................................................................85

5.3.2 Simulation of Wafer with Uniformly Distributed Pyramids.....................86

5.4 Solar Cell Results ..........................................................................................91

CONCULUSIONS................................................................................................ 101

BIBLIOGRAPHY ................................................................................................. 105

xiv

LIST OF FIGURES

FIGURES

Figure 1: Percentage of different types of PV cells production in 2013 [2] ................ 2

Figure 2: Becquerel experimental setup to show the photovoltaic effect [3] .............. 3

Figure 3: Right is multi-crystalline solar cell. Left is mono-crystalline solar cell ....... 5

Figure 4: CIGS solar cells on flexible substrate [8] ................................................... 6

Figure 5: Organic Solar Cell [14] .............................................................................. 7

Figure 6: Solar Radiation Spectrum[3] ...................................................................... 8

Figure 7: The path length in units of Air Mass, changes with the zenith angle ........... 8

Figure 8: I-V curve of a Solar Cell. Blue dash line is under dark, and red solid line is

under illumination [16] ........................................................................................... 10

Figure 9: Typical I-V curve of a solar cell............................................................... 11

Figure 10: Circuit diagram of a solar cell including the shunt resistance [3] ............ 11

Figure 11: Variation of I-V curve with respect to shunt resistance Rsh. For ideal case

Rsh=10 kΩ cm2 [3] ................................................................................................. 12

Figure 12: Variation of I-V curve with respect to series resistance Rs [3] ................ 13

Figure 13: I-V curve of a solar cell with Rs=5.1 ohm cm2 [3] .................................. 14

Figure 14: Quantum Efficiency of a solar cell [3] ................................................... 15

Figure 15 : The production steps of screen printed crystal silicon solar cell ............. 17

Figure 16: The structure of a screen printed solar cell [6] ........................................ 18

Figure 17: (a) Crystal structure of intrinsic Si; (b) P doped Si crystal structure; (c) B

doped Si crystal structure ........................................................................................ 19

Figure 18: P type and N type semiconductors respectively [3] ................................ 21

Figure 19 : P-n junction [3] ..................................................................................... 21

Figure 20:(1) Energy band diagrams of a metal and an n-type semiconductor before

combined, (2) Band diagrams after combination of a metal and an n-type

semiconductor [25] ................................................................................................. 27

xv

Figure 21: :(1) Energy band diagrams of a metal and a p-type semiconductor before

combined, (2) Band diagrams after combination of a metal and a p-type

semiconductor [25] .................................................................................................28

Figure 22: I-V curve for Ohmic and Schottky contact .............................................29

Figure 23: :(1) Energy band diagrams of a metal and an n-type semiconductor before


semiconductor for Ohmic contact [25] ....................................................................29

Figure 24: (1) Energy band diagrams of a metal and a p-type semiconductor before

combined, (2) Band diagrams after combination of a metal and a p-type

semiconductor for Ohmic contact [25] ....................................................................30

Figure 25: Solar cell operation before (1) and after (2) edge isolation [26] ..............31

Figure 26: AM 1.5 spectrum at global tilt [27] ........................................................33

Figure 27: Absorption depth of Si for 90% absorption [12] .....................................34

Figure 28: (a) The silicon crystal (b) Miller indices in a cubic structure...................35

Figure 29: Formation of square based pyramids of (100) oriented Si wafer .............39

Figure 30: Square pyramid with base length “a”, height “h” and half of the diagonal

of base “s”. The angle “α” between the pyramid edge and its projection “s” is equal

to “45o”

due to crystallographic structure [36]. ........................................................40

Figure 31: (a) Reflection from non-textured Si (b) Reflection from textured Si .......40

Figure 32: Ultrasonic cleaner ..................................................................................43

Figure 33: Hot plate and teflon holder .....................................................................44

Figure 34: Schematic of doping furnace [9] .............................................................45

Figure 35: The schematic representation of PECVD chamber [9] ............................46

Figure 36: ASYS EKRA-XL1 screen printing system ................................................47

Figure 37: Al mask for backside is in left, Ag mask for front side is in right............47

Figure 38: (a) Ideal case (b) Real case .....................................................................48

Figure 39: Finding number of pyramids by using the program ................................49

Figure 40: Wafer surface with uniform upright pyramids ........................................50

Figure 41: a) SEM image after converting black–white b) Area of black region ......51

Figure 42: Scaltec SBA31 balance ..........................................................................52

xvi

Figure 43: Reflection measurement setup [9] .......................................................... 53

Figure 44: Schematic presentation of solar simulator [9] ......................................... 54

Figure 45: Schematic representation of EQE measurement setup [9] ....................... 55

Figure 46: Reflection of textured as-cut wafers for process 5.4vol% IPA, 4.2wt%

KOH with magnetic agitation at (a) 70oC (b) 75

oC (c) 80

oC .................................... 58

Figure 47: SEM images of textured as-cut wafers with magnetic agitation with a

solution consisting of 5.4vol% IPA, 4.2wt% KOH for 15, 25, 35 and 45 min at 75 C

............................................................................................................................... 60

Figure 48: Reflection of textured SDE wafers for process 5.4vol% IPA, 4.2wt%

KOH with magnetic agitation at (a) 70o C (b) 75

o C (c) 80

o C ................................. 61

Figure 49: SEM images of textured SDE wafers with magnetic agitation with a


............................................................................................................................... 62



o C (c) 80

o C ................................. 63



............................................................................................................................... 64

Figure 52: Area covered by pyramids with respect to time and KOH concentration

for AS-CUT wafers at 75oC .................................................................................... 65

Figure 53: SEM images of samples (a) textured with 5.4wt% KOH (b) textured with

3.33wt% KOH at 80oC ........................................................................................... 65

Figure 54: : Reflection of textured SDE wafers for process 5.4vol% IPA, 3.33wt%


oC (c) 80

oC .................................... 66

Figure 55: SEM images of textured SDE wafers with magnetic agitation with a

solution consisting of 5.4vol% IPA, 3.33wt% KOH for 15, 25, 35 and 45 min at 75oC

............................................................................................................................... 67

Figure 56: Area covered by pyramids with respect to time and KOH concentration

for SDE wafers at 75oC(Lines are just added to guide the eye) ................................ 67

xvii

Figure 57: Area covered by pyramids of textured wafers with magnetic agitation with

a solution consisting of 5.4vol% IPA, 3.33wt% KOH for 15, 25, 35 and 45 min at (a)

70oC (b) 75

oC (c) 80

oC............................................................................................68

Figure 58: Reflection measurements of wafers textured with a solution consisting of

4.2wt % KOH, 5.4vol % IPA, 25 min at 75oC with and without a cap. ....................69

Figure 59: SEM images of as-cut wafers textured with a solution consisting of 5.4vol

% IPA, 4.2wt % KOH for 25 min process duration at 75oC a) without cap (coverage

21%) b) with cap (coverage 92%) ...........................................................................70

Figure 60: SEM images of SDE wafers textured with a solution consisting of 5.4vol

% IPA, 4.2wt % KOH for 25 min process duration at 75oC a) without cap (coverage

35%) b) with cap (coverage 82%) ...........................................................................70


4.2wt % KOH, 5.4vol % IPA, 45 min at 75oC with and without cap a) as-cut b) SDE

...............................................................................................................................71

Figure 62: SEM images of wafers textured with a solution consisting of 5.4vol %

IPA, 4.2wt % KOH for 45 min process duration at 75oC a) as-cut b) SDE with and

without using a cap .................................................................................................71


3.33wt % KOH, 5.4vol % IPA, 15min and 25 min at 70oC with and without cap a)

as-cut b) SDE ..........................................................................................................72

Figure 64: Reflection measurements for as-cut wafers textured with a solution

consisting of 3.33wt % KOH, 5.4vol % IPA, 15min and 25 min at 70oC with a speed

of 50 and 125 rpm ...................................................................................................73

Figure 65: Reflection measurements for SDE wafers textured with a solution

consisting of 3.33wt % KOH, 5.4vol % IPA, 25min and 35 min at 70 and 75oC with

a speed of 50 rmp and 125 rpm ...............................................................................74

Figure 66: Reflection values of as-cut wafers wafers textured with a solution

consisting of 3.33wt % KOH, different IPA concentrations a) 15min and b) 25 min at

70oC with a speed of 125 rpm .................................................................................75

xviii

Figure 67: Mass loss with respect to time and changing IPA concentration (Lines are

just added to guide the eyes) ................................................................................... 76

Figure 68: Textured wafer with a solution consisting of 40ml IPA, 3.33wt % KOH,

15 min at 70oC ........................................................................................................ 76

Figure 69: (a) Reflection values for textured as-cut wafers with magnetic agitation

(MA) and ultrasonic agitation (UA) (b) AM 1.5-weighted reflection (AM 1.5-WR)

values (c) Lowest reflection values obtained up to now(Lines are just added to guide

the eyes) ................................................................................................................. 78

Figure 70: SEM images of a) 75M_45min as-cut wafer b) 80K_45min SDE wafer

c) UA_15min as-cut wafer ...................................................................................... 79

Figure 71: Reflection measurements of as-cut wafers textured by ultrasonic agitation

at a) 70oC b) 60

oC c) AM 1.5 weighted reflection values (Lines are just added to

guide the eyes)........................................................................................................ 80

Figure 72: SEM images of as-cut wafers textured by ultrasonic agitation at a) 70oC b)

60oC ...................................................................................................................... 82

Figure 73: a) Pyramid density b) Average Pyramid height c) Mass loss for as-cut

wafers textured with ultrasonic agitation at 70 and 60oC(Lines are just added to guide

the eyes) ................................................................................................................. 83

Figure 74: Model of SDE wafer reflection simulation ............................................. 85

Figure 75: a) Reflection data b) Reflection c) Transmission d) Absorption with

respect to changing thickness (Lines are just added to guide the eyes) .................... 86

Figure 76: Simulation of textured wafer obtained by a) assembling one pyramid on a

rectangle volume b) cutting the a part of rectangle volume ..................................... 87

Figure 77: a) Uniformly textured wafer model b) Simulation model ....................... 87

Figure 78: a)Reflection values for model with 1,6,10 µm pyramids b) AM 1.5-

Weighted Reflection ............................................................................................... 88

Figure 79: Absorption graph of two side textured wafer with 6 µm pyramid height 89

Figure 80: Absorption of a) one side and two side textured wafer, b)one side wafer

with 1 µm pyramids, c) one side wafer with 6 µm pyramids d) one side wafer with

10 µm pyramids...................................................................................................... 90

xix

Figure 81: Model of a wafer with one side texturing ...............................................90

Figure 82: Graphs of Average a) Jsc b) Voc c) Fill factor e) Efficiency with respect

to texturing time d) Efficiency- Jsc, f) I-V curve of best cell ..................................93

Figure 83: Mass loss and Jsc with changing process durations a) at 60oC b) at 70

oC

.............................................................................................................................. .93

Figure 84: AM 1.5-weighted reflection and Average Jsc values with respect to

changing texturing time a) at 60oC b) 70

oC .............................................................94

Figure 85: Pseudo and real efficiencies and fill factors a),c) at 60oC b),d) at 70

oC

process temperature ................................................................................................95

Figure 86: a) J01 b) J02 with respect to texturing time ...............................................96

Figure 87: Electroluminescence images of a) Reference b) 70UA 45min solar cell..97

Figure 88: EQE and Reflection-IQE of solar cells produced by using wafers textured

for 45 min a)-c) at 60oC and b)-d) at 70

oC ...............................................................98

Figure 89: SEM image of a) 70UA 45min b) 60UA 45 min c) reference wafer after

texturing process .....................................................................................................99

xx

LIST OF TABLES

TABLES

Table 1: Parameters of three different AS-CUT/SDE wafer texturing processes using

chemical solutions based on KOH/IPA/DI-water for magnetic agitation. ................ 58


chemical solutions based on KOH/IPA/DI-water for magnetic agitation. ................ 62

Table 3: Parameters of two different AS-CUT/SDE wafer texturing processes using

chemical solutions to observe effect of using a cap ................................................. 69

Table 4: Parameters of the solutions to observe the effect of the magnetic stirring

speed ...................................................................................................................... 73


chemical solutions to observe effect of IPA concentration ...................................... 75

Table 6: Parameters of two different AS-CUT wafer texturing processes using

chemical solutions .................................................................................................. 79

Table 7: Absorption coefficient and refractive index values of mono-Si .................. 84

Table 8: Parameters of models ................................................................................ 89

Table 9: I-V measurement results by Solar Simulator ............................................. 91

Table 10: Suns-Voc measurement results ................................................................ 94

1

CHAPTER 1

INTRODUCTION

Energy is vital for human beings. The sources of energy, which are used

mostly today, are coal, natural gas, and petroleum. But these sources are limited (i.e.

not renewable), and they pollute air, water, and soil. These problems are forcing

people to find new energy sources. Some new energy sources such as solar, wind,

water, nuclear energy etc. have already been in use of people. However, some of

these sources suffer from problems such as high cost, limited availability, geological

difficulties, and environmental disturbances. For example, for a hydroelectrical

power application, there must be rivers and waterfalls available. In addition, in all

such power stains the natural environment is somewhat disturbed. Among the new

sources, solar energy is the most promising and most available energy source in the

world. Power coming from the Sun is about 174 petawatts (1 PW=1015

watts) and

51% of it is absorbed by oceans and land. In other words 89 PW can be used as an

energy source [1]. When converted to other energy types such as electricity, the solar

energy has the potential to provide all energy demand of the human being on our

planet. This conversion can be done in two ways: it can be converted to heat by

concentration and then used to generate steam for power generating turbines, or one

can use photovoltaic (PV) solar cells which convert the solar power to electricity

directly. The former one is called Solar Thermal Electricity (STE) which is outside

the scope of this study. The latter is the most widely used technology for the solar

energy conversion today. With the recent price reductions and improvements in the

performance, the PV industry has grown very rapidly in all around the world. At the

end of 2014, the total installed PV capacity exceeded 200 GWp, which is a

significant value proving the potential of PV technology in energy supply. Solar cells

which are the basic building elements of PV power applications comprises several

2

different technologies such as crystalline Si (c-Si, mono and multi), thin film

systems (a-Si, CdTe and CIGS). Among them c-Si wafer based cells are the most

commonly used solar cells in PV industry. Figure 1 compares the share of different

technologies in PV market today. From the technology and performance point of

view, we see enormous development in recent years. The conversion efficiency,

which is the major performance parameter for solar cells, has been improved with

intensive research and development activities. For wafers based Silicon (Si) solar

cells, efficiency of the cell has reached 26% which is very close to the upper

theoretical limit for these types of cells [2].

Figure 1: Percentage of different types of PV cells production in 2013 [2]

The cost of the generated electricity is a very important issue for choosing the

solar energy instead of the fossil fuels. To decrease cost per watt of solar cells, either

the cell efficiency should be increased or the cost of production should be reduced.

We observe R&D activities in both directions in many different locations around the

world. New device architecture and approaches are being designed and implemented

towards lower cost of energy generation [2]. Optimizations of the current production

steps and improving them have also been investigated intensively.

In this thesis, we focus on the texturization process of mono-crystal Si wafers,

which is the first step of producing a c-Si solar cell. The aim of texturization is to

create three dimensional structures on the surface of the wafer to reduce the

reflection, and thus improve the absorption of the solar radiation. However, forming

the best structure on a Si wafer surface with minimum material loss is still an issue

3

which needs to be addressed. By a proper optimization, one can obtain best

absorbing condition with minimum material loss. The aim of the study is to develop

a methodology to decrease the amount of removed Si, the temperature of solution,

the amount of chemical used for etching, and finally to obtain the lowest reflection

from the surface.

1.1 History of Photovoltaic Technology

Alexandre-Edmond Becquerel, a French physicist, discovered the

photovoltaic effect in 1839. Figure 2 shows his setup [3]. Two platinum (Pt)

electrodes were put in an acidic solution containing silver bromide AgBr or silver

chloride AgCl which are light sensitive material. When these light sensitive materials

are illuminated, they decompose into their ions, such as Ag+ and Cl

- ions, which are

then collected by the electrodes leading to electric current flow in the outside

circuitry.

Figure 2: Becquerel experimental setup to show the photovoltaic effect [3]

In 1876, by using solid selenium contacted with platinum, Adams and Day

observed photovoltaic effect. Then, Chapin, Fuller and Pearson used Si to produce

solar cell with an efficiency of 6% in Bell Laboratories in 1954 [4]. This was the first

4

reported solar cell produced using Si. During the development of Si solar cells, some

theoretical approaches were put forward. In 1961, the efficiency of Si solar cell being

a maximum point of 33% was found by William Shockley and Hans Queisser [5].

After that point, the development steps of production a Si solar cell can be sorted as;

In 1974, an etch solution consisting of sodium hydroxide (NaOH) was used to

texture the silicon surface, and the reflection losses due to a flat surface was

reduced [6].

In 1975, instead of using expensive vacuum metallization to produce front

Silver (Ag) and back Aluminum (Al) contacts, Ralph et al. used the screen

printing method used for the first time [6]. They produced antireflection

coatings by using titanium oxide and silicon dioxide. Their produced silicon

solar cells became the standard cell with the efficiency of about 10% in the

1970s [6].

In 1984, silicon nitride (SiNx) was used as an antireflective coating by

Kimura [6].

In 2006, 86% of wafer based solar cells were produced using screen printing

to form Ag front and Al back contacts, and chemical vapor deposition was

used to deposit SiNx as the antireflection coating on the front surface [6].

Today, the most commonly used metallization method in PV industry is the

screen printing method to produce Si solar cells, due to the low cost and practical

applications [7]. About 19% efficiencies have been obtained in industrial production

by improving the screen printing solar cells up to now.

There are three reasons why the Si solar cells are dominant in photovoltaic

industry. One of them is that Si is the second most abundant element in the Earth.

Second one is the electrical properties of Si. The electrical properties of Si are; a)

electron hole pairs can be generated under radiation, and b)Si can conduct these

electrons and/or holes, and this conductivity can be modified by introducing atoms of

other elements [6]. The last one is that Si has been used and studied for more than 40

years in semiconductor industry.

5

1.2 Types of Solar Cells

1.2.1 c-Si Wafer Based Solar Cells

Wafer based solar cells have 90% share of photovoltaic market today [2].

There are two types of wafers which are used to produce this type of solar cells. One

of them is the mono-crystalline silicon wafer. The entire wafer is in a single crystal

form edge to edge for mono-crystalline Si wafer. The other one is multi-crystalline

wafer which consists of different crystalline domains with different crystal

orientation. As can be seen from the Figure 3, one can easily distinguish which one is

mono or multi-crystalline solar cell.

Figure 3: Right is multi-crystalline solar cell. Left is mono-crystalline solar cell

Although mono-crystalline solar cells have higher efficiencies (about 24% in

lab environment), multi-crystalline solar cells, which have efficiency about 21% in

lab environment, are dominant in photovoltaic market [2]. The reason of this

situation is that multi-crystalline solar cells have lower production cost than mono-

crystalline solar cells have.

1.2.2 Thin Film Solar Cells

Thin film solar cells are produced by using a chemical or physical process to

deposit the active material on substrate. The substrate can be glass or another flexible

material as shown in Figure 4 [8]. The major thin film materials are Amorphous Si

6

(a-Si), Cupper Indium Gallium (di) Selenide (CIGS) and Cadmium Telluride (CdTe)

[9].

Figure 4: CIGS solar cells on flexible substrate [8]

By using a few micron thick material, very high absorption can be achieved.

This ability provides less material consumption during production of thin film solar

cells. But thin film solar cells have lower efficiency values compared to Si, and in

addition sunlight can spoil their forms and make them instable. These problems make

thin film solar cells less popular in photovoltaic technology when compared with c-

Si solar cell technology.

1.2.3 Other Solar Cell Types

Dye sensitized solar cells (DSSCs), tandem solar cells and organic solar cells

are examples of some other solar cell types. These new approaches are aiming to

reach low cost and/or high efficiency through non-vacuum production techniques, or

tandem structures trying to exceed the Shockley-Queisser limit. For DSSC and

organic cells, efficiencies as high as 10-11% have been demonstrated [10]. For

tandem cells, efficiencies up to 46 % have been achieved [2].

In tandem solar cells, III-V semiconductors like InAs, GaAs, InP are used and

they have the ability of absorbing different regions of solar radiation at different

layers of solar cells. This results in efficiencies up to 30.8% under 1 sun illumination

[11]. Due to the high production cost, tandem solar cells are used with systems to

7

concentrate the sun light to obtain high efficiency (about 40%) from small areas [2].

This is called Concentrated Photovoltaics (CPV).

For organic solar cells and DSSCs, their production cost is cheap, but they

have lowest efficiencies when compared with the others. Although they have low

efficiencies, their applications make them attractive. Organic solar cells can be

applied on large flexible substrates by a process based on roll to roll production

technique [12]. DSSC can be used as transparent solar cells over windows enabling a

more aesthetic and large surface integration in Building Integrated Photovoltaic

(BIPV) applications [13]. These cell types also suffer from the efficiency degradation

with time.

Figure 5: Organic Solar Cell [14]

1.3 Basic Concepts of Photovoltaic Science and Technology

1.3.1 Solar Irradiation

The sun generates huge amount of energy which is about 63 MW/m2 at its

surface. At just outside the Earth atmosphere, this energy is measured as 1360 W/m2

which is very small quantity compared to the energy at sun’s surface [15]. Due to the

atmosphere consisting water vapor, dust particles, gas molecules (carbon dioxide,

ozone), parts of the solar radiation passing through it is absorbed, reflected or

scattered. This leads to the solar intensity spectrum on the Earth’s as shown in Figure

6.

8

Figure 6: Solar Radiation Spectrum[3]

The Air Mass (A.M.) is defined as the ratio of path length of solar radiation through

the atmosphere to the shortest possible path length [3]. It is defined as;

Air Mass (A.M.) = 1/ cos(φ) (1)

Where φ is called zenith angle which is the angle between the sun and its vertical

axis.

Figure 7: The path length in units of Air Mass, changes with the zenith angle

9

Figure 7 shows A.M. 0, A.M. 1 and A.M. 1.5. A.M. 0 refers to the spectrum

just outside the atmosphere i.e. “zero atmosphere”. This quantity is used to

characterize solar cells which are used for space power applications. A.M. 1 refers to

the spectrum when the sun is at the vertical direction and A.M. 1.5 refers to the

spectrum when the sun makes an angle 48.2o with respect to the normal. Solar cells

used for commercial applications are characterized with respect to A.M. 1.5. This

optical path length results in a power of approximately 1000 W/m2

which is defined

as “1 SUN”.

1.3.2 Basics of Solar Cell Operations

A solar cell is a device which is produced by combining p-type and n-type

semiconductors like diodes. Its current-voltage characteristics are similar to diodes in

dark condition. This characteristic can be expressed by the following formula in

dark;

(2)

In the equation, I0 is reverse saturation current, q is electron charge, V is

voltage applied between the terminals of the cell, k is Boltzmann constant, T is the

cell temperature in Kelvin, and n is ideality factor which is 1 for ideal case. Under

illumination, Iop, which is the optically generated current, term is subtracted from the

Equation 2 and the equation becomes;

(3)

The Figure 8 shows I-V curve for a solar cell under dark and illumination condition.

10

Figure 8: I-V curve of a Solar Cell. Blue dash line is under dark, and red solid line is

under illumination [16]

To get the conventional I-V curve of solar cell, red curve is mirrored about horizontal

axis and the 1st quadrant is taken.

Short Circuit Current (Isc)

The short circuit current is defined as the point which I-V curve intersects the

I-axis. In other words, Isc is obtained when the applied voltage between the terminals

of solar cell is zero or equal. From the Equation 3, one can easily say that Isc equal to

Iop. This is the maximum current which can be generated by a solar cell.

Open Circuit Voltage (Voc)

The open circuit voltage is defined as the point which I-V curve intersects the

V-axis. In other words, the net flowing current through the solar cell is zero. From

Equation 3, Voc is the maximum voltage which is generated by a solar cell and can be

obtained as;

(4)

The Figure 9 illustrates the typical I-V curve of a solar cell.

11

Figure 9: Typical I-V curve of a solar cell

Shunt Resistance (Rsh)

Shunt resistance is one of the main reasons of the power loss in solar cells. It

is mainly due to easy current channels due to the defects across the junction region.

When Rsh is low, the generated current follows these current channels leading to

recombination, and thus reduction of the open circuit voltage across (Voc) the solar

cell. This causes a reduction in the generated power. This situation is illustrated in

the following figures; Figure 10 and Figure 11.

Figure 10: Circuit diagram of a solar cell including the shunt resistance [3]

12

Figure 11: Variation of I-V curve with respect to shunt resistance Rsh.

For ideal case Rsh=10 kΩ cm2 [3]

One can understand from the Figures that to get high power from a solar cell Rsh

should be as high as possible. Shunt resistance can be expressed as [3];

(5)

which is easily determined from the slope of the I-V at V=0.

Series Resistance (Rs)

There are three main factors which form the series resistance for a solar cell

namely: the movement of current through the emitter and base of the solar cell, the

contact resistance between the metal contact and the silicon, and the resistance of the

top and rear metal contacts [17]. As the Rs increases, the fill factor decreases, and Isc

also decreases for higher values of Rs as shown in the Figure 12.

13

Figure 12: Variation of I-V curve with respect to series resistance Rs [3]

For ideal case, the series resistance I should be zero. The series resistance can be

expressed as [3];

(6)

The series resistance is calculated from the slope of the I-V curve at V = Voc.

Fill Factor (FF)

Fill factor is the ratio of maximum power to the product of short circuit

current and open circuit voltage. The voltage at maximum power is shown as Vmp,

and the current at maximum power is shown as Imp. The fill factor is defined as;

(7)

The fill factor is also defined as the ratio of the area A to area B as shown in the

Figure 13.

14

Figure 13: I-V curve of a solar cell with Rs=5.1 ohm cm2 [3]

Conversion Efficiency

The efficiency of a solar cell is defined as the ratio of the maximum output

power which is maximum power as shown in Figure 13 to the input power, and it is

commonly used parameter to identify which cell is better than the other. The

efficiency is expressed as;

(8)

Here to define Pin, under which condition the efficiency is measured must be

considered. For commercial solar cell, A.M. 1.5 conditions are used. For this

condition, Pin is defined as the area of the solar cell multiplied by 1kW/m2. This

value is equal to 24.3 W for a 156 × 156 mm2 cell.

Quantum Efficiency (Q.E.)

Quantum efficiency is the ratio of the number of collected carriers by the

solar cell to the number of photons for a given energy incident on the solar cell [18].

15

If all incoming photons at a certain wavelength is absorbed, and all the generated

minority carriers are collected, the quantum efficiency at that wavelength will be 1 as

shown in the Figure 14.

Figure 14: Quantum Efficiency of a solar cell [3]

Recombination effects are responsible for the reduction in quantum efficiency

of a solar cell. For example blue light is absorbed near the surface, and if the front

surface passivation is not good, the quantum efficiency is reduced about these

wavelengths as shown in Figure 14. For silicon, silicon is transparent below 1.12 eV

which is the energy band gap of silicon and quantum efficiency is zero for

wavelengths beyond 1107 nm.

Spectral Response

Spectral response (S.R.) is a parameter which is the ratio of the current

generated by the solar cell to the power incident on the solar cell [3]. It is expressed

as;

(9)

17

CHAPTER 2

FUNDAMENTALS OF CRYSTALLINE SILICON SOLAR CELL

TECHNOLOGY

2.1 Introduction

Screen printing method is the most commonly used method to produce silicon

based solar cells by solar cell industry. The production steps are shown in Figure 15.

Figure 15 : The production steps of screen printed crystal silicon solar cell

Texturing is the first step of the production steps to form upright pyramids on

a Si-wafer, then a doping process is used to obtain p-n junction, after that an anti-

reflective coating which is a thin film of SiNx is made to passivate the emitter and

reduce the reflection. Then the screen printing method is used to deposit front and

back contacts, and electrical contacts between the Si wafer and the pastes are

established by a firing process. Finally, edge isolation is made to isolate the front

from the back side of the wafer, and characterization of the solar cell is conducted.

Figure 16 shows the structure of such a solar cell [6].

18

Figure 16: The structure of a screen printed solar cell [6]

2.2 Texture: Wet Chemical Texture

Texturing of a silicon wafer is the first process of production of screen printed

solar cells to reduce the reflection losses by producing pyramids on the surface of the

silicon wafer. The process is carried out by using an alkaline solution which consists

of potassium hydroxide (KOH) or sodium hydroxide (NaOH), isopropyl alcohol

(IPA) and de-ionized water (DI-water) which is heated to 75-80oC, and wafers are

put in it up to a certain time. This solution is known as KOH-IPA solution in the

photovoltaic industry.

The anisotropy of the etch solution is used to obtain pyramids on the wafer.

This anisotropic behavior of the solution is due to different crystal orientations

having different etch rates. KOH or NaOH etches (100) and (110) surfaces of the Si-

crystal with a higher rate with respect to (111) surface [19]. Coverage of the surface

with pyramids is obtained by optimizing temperature, time, concentrations of DI-

water, IPA and KOH or NaOH. In chapter 3, texturing process will be covered in

detail.

2.3 Doping

Doping is the second process of production of screen printed solar cells.

Before doping, a pre-cleaning step is applied for textured wafers in order to remove

all inorganic-organic contaminants and the natural oxide that may have grown on the

19

wafer surface by using DI-water, hydrofluoric acid (HF), hydrochloric acid (HCl)

and cleaned wafers are dried in hot air.

In general, doping is used to change the electrical properties of

semiconductors like conductivity, resistivity etc. by adding impurity atoms into

crystalline lattice. In solar cell industry, it is done by solid state diffusion process.

This process takes place into a tube shaped oven heated to 800–900oC to provide

enough energy in order to change a lattice atom with an impurity atom.

Silicon is an element of Group IV-A of periodic table and its atomic number

is 14. The Silicon electron configuration is 1s22s

22p

63s

23p

2. It means that a silicon

atom has to make four bonds to be stable as shown in Figure 17 a. When a V-A

group atom having five valance electrons (in solar industry, phosphorous (P) is used)

is added to obtain n type silicon, the P atom makes 4 bonds with the nearest Si atom

and the rest electron of it will be weakly bonded to it as shown in Figure 17 b. The

weekly bonded electron can be separated easily even at room temperature to move

through the lattice, and it makes the conductivity higher than before.

Figure 17: (a) Crystal structure of intrinsic Si; (b) P doped Si crystal structure; (c) B

doped Si crystal structure

An element of III-A group like Boron (B) having three valance electrons is

doped; there will be a missing electron which is equivalent to an additional hole, in

the structure as shown in Figure 17 c. More holes make the conductivity higher than

before. This type of material is called as p-type Si.

20

Standard solar cells are produced on p-type solar cells. N type layer is formed

by using liquid phosphorous oxychloride POCl3. POCl3 is converted into gas form by

nitrogen gas which flows through the liquid, and it is sent into a tube furnace at a

temperature about 840oC. During the flow of this mixture O2 gas is also sent to the

furnace, and the following reactions take place;

4POCl3(g) + 3O2(g) 2P2O5(g)+6Cl2 (10) [20]

2P2O5(g)+5Si(s)5SiO2(s)+4P(s) (11) [20]

xSiO2+yP2O5xSiO2.yP2O5 (12) [20]

In the first reaction, POCl3 reacts with O2, and P2O5 is formed. This reaction

occurs in predeposition step. Then, P2O5 reacts with Si, and SiO2 is formed on the

surface of the wafer, and P atoms are released as shown in the second reaction. P

atoms diffuse into Si crystal and replace Si atoms, and the p type region is converted

into n type. This reaction occurs in drive-in step. In the third reaction, P2O5 reacts

with SiO2, and the phosphorous silicate glass (PSG) is formed. After doping, PSG is

removed by using HF solution.

2.3.1 P-n Junction

As mentioned in the previous section, majority carriers are holes and

electrons, in p type and n type respectively. There are also ionized and fixed atoms in

crystal lattice which are negatively charged (like B-) in p type and positively charged

(like P+) in n type as shown in Figure 18.

21

Figure 18: P type and N type semiconductors respectively [3]

When p type and n type semiconductors are combined, electrons from the n-

type region diffuse to p type region and leave behind uncompensated positive ions.

Similarly, holes in p-type diffuse to n type and leave behind uncompensated negative

ions. The uncompensated ions generate an E-field, which prevents substantially the

diffusion of e- to p type and h

+ to n type in equilibrium condition. The Figure 19

illustrates that situation;

Figure 19 : P-n junction [3]

22

The region in which there are no free carriers is called depletion region. Also,

thanks to E field, the minority carriers (h+ for n type and e

- for p type) which are

close to the depletion region or generated within a diffusion length distance can drift

to the other side and be collected easily. And a "built in" potential Vbi due to E

field is formed at the junction.

2.3.2 Modeling of Solid State Diffusion

The doping process takes places by solid state diffusion of the dopant atoms.

This process is explained by Fick’s theory of diffusion which was derived by Adolf

Fick in 1855. The Fick’s first law is [21];

(13)

J is the diffusion flux which is defined as the amount of solute transferred per

unit area per unit time and has the units of mol/cm2.s.

D is diffusion coefficient or diffusivity of the material defining how easily an

atom could move throughout a known medium. Diffusivity defined with the

units of cm2/s , changes as a function of temperature and concentration.

C(x,t) is the concentration of atoms per unit volume of the medium and

defined with the units of mol/cm3.

is known as the concentration gradient which is the driving force of

the diffusion process.

The matter flows in the direction of decreasing solute concentration is stated

by using the negative sign in the eq. 13. In addition, spatial change in diffusion flux

must be equal to the change in solute concentration per unit time within a very small

volume of the system as imposed by the continuity. This situation is expressed in Eq.

14 [21];

(14)

23

Inserting eq. 13 into eq. 14, the Fick’s 2nd

law of Diffusion for one

dimensional flow is obtained as shown in eq. 15 [21];

(15)

Eq. 15 can be solved by using different boundary conditions. One of them is

that constant source concentration at the surface corresponding to the initial

condition C(x,0) = 0 and boundary conditions C(0,t)= Cs and C(∞ ,t) =0 leads to a

solution in the form of complementary error function as expressed in Eq. 16 below

where Cs is the surface concentration of dopant atoms [21].This situation formulize

the predeposition step of the diffusion process in which dopant including process gas

is fed to the system with a constant flow rate.

(16)

The drive in process takes place after predeposition step. In this process, a

thin layer of dopant atoms is already present on the wafer surface. For this process,

initial condition is C(x,0)=0 and the boundary conditions are ∞

and

C(∞, t) =0 , where QT is the total amount of dopant per unit area and will diffuse into

the Si. The resultant solution satisfying the initial and boundary conditions will be in

the form of Gaussian distribution function whose mathematical expression is given in

eq. 17 [21].

(17)

24

2.4 Anti Reflecting Coating (ARC)

The surface of the cell is usually covered by SiNx layer as anti reflection

coating. Deposition of the SiNx:H layer is done by using plasma-enhanced chemical

vapor deposition (PECVD). There are two significant reasons of deposing a SiNx:H

layer on the front side of the wafer. One of them is to reduce the reflection losses.

The reflection index of the SiNx:H layer is about 2 which is between that of Si (3.5)

and that of air ( approximately 1) for a wavelength of 600 nm. This makes the layer

working well as an antireflective coating. So, the wafer with SiNx has lower

reflection comparing with that of the bare wafer, and more light can be absorbed in

the solar cell [6]. The other reason is the passivation effect of the layer on the

emitter. The SiNx:H layer is very rich in hydrogen H, hydrogen also diffuses into the

Si-wafer and passivates some impurities or crystal defects [6].

As a passivation and antireflective layer, silicon dioxide (SiO2) can also be

used. However, use of SiO2 has two disadvantages. The first one is that the

processing temperature (about 1000oC) is high, and it does not passivate as efficient

as SiNx:H. The other one is the refraction index of SiO2 (1.5) is low.

Theory

Optical behavior of a surface is related to the dielectric constant. Dielectric

constant can be described as in eq. 18 [22];

(18)

In eq. 18, is dielectric constant, is real part of the refractive index, and

is the imaginary part of the refractive index, which is used to define absorption

coefficient ( )as shown in eq. 19 [22].

(19)

25

Reflectance for the light travelling from a medium with refractive index n0 to

a medium with refractive index ns at normal incidence is expressed as in eq. 20 [22];

(20)

If the material is coated with an ARC thin film with refractive index n1, the

eq. 20 becomes for the light of wavelength [22];

(21)

where is defined as the phase shift in the film with thickness d1 and can be

expressed as in eq. 22 [22];

(22)

θ1 is the angle between the light ray and the normal of the film surface.

When θ1 is zero (i.e. light at normal incidence), the minimum reflection is obtained at

which leads to eq. 23;

(23)

26

To minimize the reflection once more, the light reflected from the rear and

front sides of the thin film should be out of phase so interfere destructively [23]. To

get this situation, n1 can be chosen to satisfy the eq. 24 below [22].

(23)

By using the equations above, for a single wavelength the reflection can be

made zero. Since the peak power of the solar spectrum is close to wavelength 600

nm, refractive index and thickness of ARC thin film is chosen to minimize reflection

at this wavelength in photovoltaic industry [24]. At 600 nm, refractive index of Si is

ns= 3.939, air refractive index is n0=1. Therefore n1 can be chosen as approximately

2, and d1 can be chosen as 75 nm.

2.5 Metallization Screen Printing

Generated minority carriers in the depletion region and within the diffusion

length distance from the depletion region are collected by metal contacts which are

silver contacts on front and aluminum contacts on rear of the wafer. These contacts

are applied using screen printing method.

To get Ag metal in physical contacts with the front side of the cell, the paste

containing lead borosilicate glass (PbO-B2O3-SiO2) and organic compounds is used.

During the firing process, the lead borosilicate glass etches the ARC film (SiNx thin

film) and provides adhesion of Ag contacts. The firing process takes place at around

875oC .

For Al back contact, a similar process takes place. To overcompensate the

phosphorous doping on the backside and to form proper Back Surface Field (BSF),

the quantity of paste is very important [6].

The temperature of firing process must be adjusted to get good and optimal

solar cells. For temperatures below 800oC, good electrical contact between pastes

and silicon does not form, and this leads very high series resistance [6]. For

27

temperatures higher than 900oC, the pastes can diffuse too deeply into silicon and

cause short circuits between the front and back contacts [6].

When a metal and a semiconductor are combined, the electrical contact forms

in two ways; Schottky and Ohmic contact. These contacts are related to whether the

work function of metal is grater or less than the semi conductor.

Schottky Contact

When a metal and a semiconductor are combined, charge transfer occurs until

the Fermi levels get the same position. To understand this situation, we look energy

band diagrams shown in Figure 20.

Figure 20:(1) Energy band diagrams of a metal and an n-type semiconductor before


semiconductor [25]

In the Figure above, is the electron affinity, Φm and Φs are work function

for metal and semiconductor respectively. Φm must be greater than Φs for

combination of metal and n-type semiconductor to get Schottky contact. As shown in

Figure 20 (2) conduction and valance band of semiconductor will bend downwards

28

to prevent further electron diffusion from semiconductor to metal after combination of

the materials. Schottky barrier (ΦB) is formed and electron flow is stopped. Due to

electron flow to metal, there are uncompensated donor ions, and these ions induce

negative charges at metal side as shown in the Figure 20.

Figure 21: :(1) Energy band diagrams of a metal and a p-type semiconductor before

combined, (2) Band diagrams after combination of a metal and a p-type semiconductor

[25]

Figure 21 shows the energy band diagrams for a metal and a p-type

semiconductor before and after the combination. To form Schottky barrier for a

metal and p-type semiconductor, Φm must be less than Φs. The bending of the

conduction/valance band is upward to prevent hole flow between two sides.

Ohmic Contact

For Ohmic contacts, there is no or negligible potential barrier being formed

by combination of a metal and a semiconductor. In this case, I-V characteristic is

linear and independent of the polarity of applied voltage [25] as shown in Figure 22.

29

Figure 22: I-V curve for Ohmic and Schottky contact

For ohmic contact, relation between work functions is reversed. In other

words, Φm must be less than Φs for n-type semiconductor-metal combination, and Φm

must be greater than Φs for p-type semiconductor-metal combination. Energy band

diagrams are shown for n-type and p-type in Figure 23 and Figure 24 respectively.

Figure 23: :(1) Energy band diagrams of a metal and an n-type semiconductor before


semiconductor for Ohmic contact [25]

30

Figure 24: (1) Energy band diagrams of a metal and a p-type semiconductor before

combined, (2) Band diagrams after combination of a metal and a p-type semiconductor

for Ohmic contact [25]

Although current is allowed in one direction due to the potential barrier for

Schottky contact, current can flow in both directions for Ohmic contact. Therefore,

front and back contact of solar cells are required to be Ohmic. As mentioned above,

Ag is used for front side for n type region, and Al is used for back side for p type

region to get Ohmic contact.

2.6 Edge Isolation

During the doping process, front side, back side and edges of p-type silicon wafer is

doped to produce n-type region. Although Al is diffused and converts the n-type

region formed at backside into highly doped p-type (p+) region in metallization

process, edges are not converted. This situation forms possible paths for generated

minority carriers to be able to flow to p-side through the edges, instead of being

collected at an external load. Figure 25-1 on the next page illustrates this situation.

Edge isolation disconnects the electrical contact between the front and the back

contacts. This is accomplished by either plasma/chemical etching which removes n

31

regions on edges completely or by laser ablation which forms shallow grooves on the

solar cell. Figure 25-2 shows the operation of solar cell after edge isolation.

Finally, the solar cells current-voltage characteristics of solar cells are

obtained by measurements under AM1.5 spectrum illumination conditions in order to

characterize.

Figure 25: Solar cell operation before (1) and after (2) edge isolation [26]

33

CHAPTER 3

FUNDAMENTALS OF CHEMICAL ETCHING PROCESS AND

LIGHT TRAPPING

3.1 Introduction

Absorption of photon is the critical process for solar cell devices because

absorbed photons generate electron hole pairs which form the output power of the

solar cells. As mentioned above, the band gap of silicon is 1.12 eV, and wavelength

between 300-1100 nm is suitable for the photovoltaic conversion as shown in Figure

26.

Figure 26: AM 1.5 spectrum at global tilt [27]

34

The Figure 27 shows the thickness which is needed to absorb the incoming

wavelength. As can be seen from the Figure, wafers with 400 µm thickness are

needed to absorb 90 % of incoming intensity with wavelength between 300-1000 nm.

Figure 27: Absorption depth of Si for 90% absorption [12]

Use of thicker wafers causes problems like difficulties in collection of

generated electron-hole pairs having minority carrier diffusion lengths 14 µm for n

type and 140 µm for p type [23], heavy modules, high cost etc. In other words,

thicker wafers have high absorption but they have low collection probability whereas

thinner wafers have high collection probability whereas they have poor absorption at

long wavelengths. To avoid these problems, surface texturing is used.

Although there are several dry texturing process techniques reported in the

literature to produce low reflective textured surfaces, structures produced by dry

process like needle, tips, wires etc. are not suitable for reliable passivation of c-Si

surface in solar cells [28]. On the other hand, pyramids formed as a result of

different etch rates of (100) and (111) planes are of the orders of micrometer scale.

These properties of alkaline texture make it an essential step for the industrial mono-

c applications.

35

Figure 28 (a) shows the crystalline structure of Si which is a face centered

cubic structure with a base of two identical atoms located at the (0, 0, 0) and (1/4,

1/4, 1/4) position. Every silicon atom is located at the center of a tetrahedron and is

covalently bonded to four other atoms. The number of free bonds of silicon atoms is

the difference in surface planes which are represented by Miller indices as shown in

Figure 28 (b). Si atoms have one free bond in the (111) plane, which means Si atoms

in this plane are bounded to the Si crystal with three other Si atoms. On the other

hand, Si atoms in the other planes, like (100) and (110), are bounded with two other

Si atoms and have two free bonds. Therefore, energy needed to dissolve Si atoms in

(111) plane is higher than energy required to dissolve atoms in (100) and (110)

planes. The energy differences lead that texturing of Si wafer with KOH-IPA

solution which is an anisotropic etching process (i.e. etching rate is orientation

dependent). In the next section, explanation of the etching process is given [29].

(a)

(b)

Figure 28: (a) The silicon crystal (b) Miller indices in a cubic structure

36

3.2 Wet Chemical Etching with KOH/IPA Solution

Although there are many explanations [29]–[32] about the anisotropic and

selectivity effects of alkaline etch solutions on mono-crystalline silicon, the process

keeps its mystery, and further investigations should be done [6] . Among these

explanations, H. Seidel et al. [29] described well the mechanism of the etching

process [6]. In this section, the KOH based etching process is explained by using

Seidel explanations.

Two hydroxide ions OH- from the alkaline solution bind to Si atoms having

two free bonds on plane (100) and two electrons from OH- are injected in the

conduction band of the silicon as indicated in Equation 25.

(25)

The strength of Si-Si back bonds weakens due to the bonds between Si and

OH. A soluble silicon hydroxide (Si-OH) complex is obtained by breaking the Si-Si

back bonds of the Si(OH)2 which happens when two electrons from back bonds are

excited to the silicon conduction band. This is shown in Equation 26. The reactions

shown in Equations (25) & (26) occur more or less simultaneously.

(26)

The positive silicon hydroxide complex reacts with another two hydroxide

ions from the solution and orthosilicic acid Si(OH)4 is produced as shown in

Equation 27.

37

(27)

The Si(OH)4 can now get mixed into the solution by diffusion, and it is

unstable due to the high pH value greater than 12 of the solution. For these pH

values, the detachment of two protons from Si(OH)4 ,which forms the following

complex as shown in Equations 28 & 29, will occur. This silicate species was

observed by Raman spectroscopy measurements [31].

Si (OH) 4 Si O 2(OH) 2--

+ 2 H+ (28)

2 H+ + 2 OH

-2 H2O (29)

The electrons located near the surface from Equations (25) & (26) in the

conduction band can be transferred to the water molecules close to the surface of

silicon, thus hydroxide ions and hydrogen atoms are produced, and they recombine

to molecular hydrogen as shown in Equations (30) & (31).

4 H2O + 4 e- 4 H2O

- (30)

4 H2O- 4 OH

- + 4 H 4 OH

- + 2 H2 (31)

Si atoms on (111) plane have one free bond and react with one hydroxide ion as

shown in Equation (32).

(32)

Three back bonds of the surface silicon atom have to be broken. In analogy to

Equations (26) & (27), this will happen when three electrons are transferred to the

38

conduction band , and three OH- will bind to Si-OH complex as shown in Equations

(33) & (34). The reaction will continue as described above for a (100) plane.

(33)

[Si-OH]+++

+ 3 OH- Si(OH)4 (34)

According to Seidel study, there is no significant role of the potassium cations

K+ and IPA in the etching process of Si, and they can be neglected during forming

reaction equations [6].

The effects of IPA found in the literature in the etching process can be listed

below;

As the concentration of IPA increases, the etch rate decreases [33].

Its role is to move away H2 bubbles from surface by decreasing the silicon

surface tension and improving the wettability of the wafer surface [34], [35].

When this happen, the attraction between the etching solution and the surface

will form easily, and homogenous texturing will occur.

IPA adjusts the relative water concentration of the etchant [29]. It attracts

water concentration around itself, so it regulates the etching rate for solutions

with a constant KOH concentration and constant temperature [6].

3.3 Surface Morphology and Light Trapping

Due to the difference in the etching rates of different planes, square based

pyramids are formed during the etching process. The

Figure 29 shows the steps of formation of pyramids. A wafer with (100) orientation

is dipped into the KOH/IPA solution and etching starts. As mentioned above, (100)

39

plane is etched faster than (111) plane and the proportionality between them is 600/1.

The process continues with high etch rate until all surfaces become (111) planes.

After that point, the process will be very slow; i.e. no change will be observed.

Figure 29: Formation of square based pyramids of (100) oriented Si wafer

The pyramids, whose bases are formed by (100) planes and sides are formed

with (111) planes, will be formed on the surface of the mono-Si wafer at the end of

the process.

To observe surface morphology of textured wafers, scanning electron

microscope (SEM) is used, because SEM pictures have strong signal contrast, lateral

resolution and depth of sharpness [36]. By using SEM pictures, the edge of pyramids

can be measured and the height of the pyramids (pyramid size referred in the

literature) can be determined as shown in the Figure 30. As shown in the Figure, the

height of the pyramids is equal to “s” which is equal to half of the edge.

40

Figure 30: Square pyramid with base length “a”, height “h” and half of the diagonal of

base “s”. The angle “α” between the pyramid edge and its projection “s” is equal to

“45o”

due to crystallographic structure [36].

The formation of the pyramids on the wafer increases the optical path length

which is the distance being travelled by a photon without being absorbed. In other

words, texturing process makes silicon thicker optically without changing the actual

thickness of it and increases the probability of photon absorption. The Figure 31

illustrates this situation.

Figure 31: (a) Reflection from non-textured Si (b) Reflection from textured Si

41

As shown in the Figure 31 (a), the incoming light hits the surface and goes on

its path in the Si. It reaches the back side and leaves the Si. In this case huge amount

of light is reflected from the surface and leaves without being absorbed.

The Figure 31 (b) shows that incident light striking the textured surface. In

this case, some amount of incident light will be transmitted into wafers and some will

be reflected. Therefore, square of non-textured wafer reflection will be the limit one

can reach .The probability for more than one bounce of light depends on the angle

“Ø” (equal to 54.7o

in our case) which is the angle between Si surface (in our case

(100) plane) and the faces of the pyramids (in our case (111) plane) [37]. If the angle

“Ø” is less than 30o, the normally incident light will not receive more than one

bounce. If Ø is between 45o

and 60o, the double bounce of normally incident light

will occur. If Ø is greater than 60o, the triple bounce of light will occur [37]. When

the reflected light reaches the other pyramid, some amount of it will be again

reflected and transmitted. Therefore, the transmitted light into the wafer will be

increased and the reflection will be decreased. In the wafer, the light will not leave as

in the first case and it will be reflected more than one and the optical path will be

increased. As the optical path increases inside the wafer, more light will be absorbed

and more electron hole pair will be generated. In other words, the efficiency will

increase.

43

CHAPTER 4

EXPERIMENTAL PROCEDURES

4.1 PROCESS FLOW OF SOLAR CELL FABRICATION

4.1.1 Texturing of Silicon Wafer

Process flow of Si solar cell production starts with texturing process. An

ultrasonic bath shown in Figure 32 is used to agitate the KOH-IPA solution, and its

frequency is 35 kHz. It is capable of processing 25 wafers per run. Ultrasonic source

is used to get uniform temperature distribution and solution homogeneity during

reaction. The percentage of ingredients will be mentioned in chapter 5.

Figure 32: Ultrasonic cleaner

44

For magnetic stirring texturing process, a hot plate, teflon holders a glass

beaker (2.5 liter volume) shown in Figure 33 are used. The hot plate has a stirring

speed controller and temperature controller.

Figure 33: Hot plate and teflon holder

The process is followed by rinsing the textured wafers with DI-water,

cleaning in HF-HCL solution and drying them in hot nitrogen gas. While removing

metal impurities on the Si-wafer surface is done by HCl, the native oxide and the

impurities on surface are removed by HF which makes the wafer surface free of trace

metals [33].

4.1.2 Doping

Standard doping recipe optimized at GUNAM Laboratories is used to get 55

Ω/ sheet resistance. Doping process is done by using SEMCO Engineering

Incorporation MINILAB Series doping furnace. The dopant source is liquid

45

phosphorous oxychloride POCl3 transported into furnace by N2. Figure 34 shows the

schematic of doping furnace [9]. The process flow is mentioned in the section 2.3.

Figure 34: Schematic of doping furnace [9]

4.1.3 Anti-Reflecting Coating

By using SEMCO Engineering Incorporation MINILAB Series, a-SiNx

deposition is carried out in PECVD chamber. The chamber can take 10 wafers per

run. The schematic representation of the chamber is shown in Figure 35.

46

Figure 35: The schematic representation of PECVD chamber [9]

The chamber is filled with SiH4 and NH3 gases. The process is carried out at

380oC and 1 Torr. An RF generator, which works at 375 W with 50 kHz, is used to

obtain plasma. The chemical reaction of process is shown in Equation 35.

3SiH4(g) + 4NH3(g) Si3N4(s) + 12H2(g) (35)

4.1.4 Metallization and Co-Firing

ASYS EKRA-XL1 screen printing system has been used for metallization of

textured, doped, and coated with ARC Si-wafers. The system is in GÜNAM

laboratory and is shown in Figure 36.

47

Figure 36: ASYS EKRA-XL1 screen printing system

Backside is covered fully with Al, and front side is covered using a

metallization mask with Ag as mentioned in the section 2.5. The design of mask is

done by considering two issues. One of them is the shadowing effect. The busbar’s

and finger’s width should be as small as possible to obtain more interaction between

incoming light and front surface of the wafer. As mention above, more light means

more short circuit current. The design of back and front contacts are shown in Figure

37. The other one is to obtain lowest resistance, because the contact resistance

increases, the efficiency decreases.

Figure 37: Al mask for backside is in left, Ag mask for front side is in right

48

To overcome these two requirements, the width and height of the finger

shown in Figure 38 should be optimized. The ratio of the width and height is called

aspect ratio. The width of finger should be as small as possible to overcome

shadowing effect, and the height of finger should be as long as possible to get large

finger cross sectional area which leads less resistivity. The aspect ratio can be

between 0.20 and 0.30 in screen printing metallization [38].

Figure 38: (a) Ideal case (b) Real case

After finding the optimized aspect ratio and printing front contact and back

contact, the contacts are dried. Wafers with dried contacts are ready to co-firing

process by using a conveyor belt. During the process, the wafers are exposed to high

temperature under atmospheric condition. The temperature profile of the firing

furnace is like a sharp Gaussian distribution function with a peak temperature around

830oC. At the end of process, Al is melted and forms p

+ back surface field, and Ag

diffuses through the ARC layer and forms front contact without melting.

4.1.5 Edge Isolation

The last process for producing a Si-solar cell is the edge isolation. The

process is carried out by using an IR marking laser. The edges of wafer are scanned.

At the end of the process, 10-15 µm deep grooves are formed and the connection

between the back and front (p and n region) is interrupted.

49

4.2 Average Pyramid Heights and Pyramid Density, Mass Loss

By using SEM images, two parameters namely; average pyramid heights

(APH) and pyramid density (PD) can be found. To find these parameters, I used an

open source program “Imagej”. The program can identify the tips of the pyramids

and give the number of pyramids in the SEM images as shown in Figure 39. Also by

using the program, the area of the picture can be obtained. By using these quantities,

PD is equal to number of pyramids divided by area of image as shown in Equation

36.

Figure 39: Finding number of pyramids by using the program

(36)

To find the average pyramid height, it is assumed that the bases of pyramids

do not nest each other and the surface is covered with pyramids as shown in

50

Figure 40. As mentioned in section 3.3, the height of the pyramid is equal to “s”.

APH can be found by using the equation 37, 38 and 39.

Figure 40: Wafer surface with uniform upright pyramids

(37)

(38)

(39)

Using “ImageJ”, the area covered by pyramids can be estimated from the

SEM images. The image is converted to black-white image as shown in Figure 41(a)

51

where the black area is non-textured surface and the white area is surface with

pyramids. Then, the black-white image is analyzed by the program that gives area of

the black region as shown in Figure 41(b). By subtracting this quantity from 100, the

textured area can be obtained.

(a)

(b)

Figure 41: a) SEM image after converting black–white b) Area of black region

Mass loss of textured wafers is measured by using a balance, Scaltec SBA31

shown in Figure 42. The mass loss is calculated by measuring the weight of wafer

before and after texturing process and taking the ratio.

52

Figure 42: Scaltec SBA31 balance

4.3 Characterization of Solar Cells

4.3.1 Reflection Measurements

First characterization step is reflection measurement of textured wafers.

Reflection of a wafer shows the quality of the texturing process. In other words, as

the area without pyramids increases, the reflection increases. Also reflection of

wafers with ARC is measured.

The measurement setup is shown in Figure 43. The setup consists of an

integrating sphere, a lock-in amplifier, a chopper and controller, a calibrated silicon

detector, a halogen lamp, a monochromator, a focusing lens and a PC.

53

Figure 43: Reflection measurement setup [9]

Light coming from the halogen lamp is modulated by the chopper whose

frequency is not equal to multiples of 50 Hz to prevent the affects of network

electricity. Frequencies which are not equal to chopper’s frequency are filtered by the

connection between chopper controller and lock-in amplifier. After that, the spot of

modulated light is decreased by the diaphragm and focused on the wafer surface by

the converging lens. The reflected light is collected at the entrance slit of

monochromator after many reflections. The light passing through the

monochromator is detected by the Si-detector which is connected to the computer via

the lock-in amplifier. The reflection is measured between 350 nm and 1100 nm. To

calculate the reflection of the sample, the (total) reflectance of BaSO4 reference disk

that is used in (our regular) total reflectance measurements has been measured using

NIST traceable Spectralon Diffuse Reflectance Standard (WS-1-SL) from Labsphere

[39]. Therefore, the reflection of the sample will be equal to the ratio of the sample’s

intensity to that of calibration disk.

54

4.3.2 I-V Measurements

The measurement is carried out in GÜNAM laboratory by using AM1.5G

calibrated QUICK SUN-120CA-XL Solar Simulator which is shown schematically in

Figure 44. Simulator’s software provides cell efficiency, fill factor, series resistance,

shunt resistance, open circuit voltage, short circuit current and maximum power point

as output data.

Figure 44: Schematic presentation of solar simulator [9]

4.3.3 Electroluminescence Measurements

The measurement is carried out by using a MBJ closed box

Electroluminescence system in GÜNAM laboratory. The measurement is done by

applying bias to the cells to force them to radiate. No radiation will be observed in

dead regions where radiation does not occur and the generated picture gives us

information about cracks and contacts problems.

55

4.3.4 External Quantum Efficiency (EQE) Measurements

In this measurement, the number of electrons generated per incident photon

as a function of wavelength is determined. EQE measurement provides information

about the effects of contacts, the junction quality, and recombination dynamics

across the device. The setup is shown in Figure 45.

Figure 45: Schematic representation of EQE measurement setup [9]

The light is modulated by the chopper and passes through the

monochoromator. Then it is focused on the wafer between the fingers by the

converging lens. The generated electron hole pairs are collected by two probes which

are connected to the front Ag busbar and Al back surface. The resultant response is

monitored after the collected signal amplification by lock-in amplifier.

56

4.3.5 Suns-Voc Measurements

The measurement is carried out using Suns-Voc system in GÜNAM

laboratory. The results of the measurement give us information about the base

material and passivation quality. In addition, the I-V behavior of the cell can be

predicted by analyzing the obtained data under zero series resistance assumption

which leads to estimate some important cell parameters like pseudo efficiency,

pseudo fill factor etc., because the effect of shunt and series resistance could have

been separated from each other. Power loses due to series resistance can be estimated

by comparing efficiency measurement results obtained by using solar simulator and

Suns-Voc [40].

57

CHAPTER 5

RESULTS AND DISCUSSION

5.1 Texturing Process with Magnetic Agitation

In the first part of the study, the effects of temperature, time, and

concentration of the ingredients were investigated by using the hot plate. In addition,

the effect of surface morphology was investigated by using as-cut and saw damage

etched (SDE) wafers. In this part, 5x5 cm2, p-type, 1-3 Ω-cm, (100) CZ-Si wafers

were textured. After texturing process, reflection measurements and SEM images

were taken to obtain texture quality and surface morphology.

5.1.1 Effect of Process Duration

To observe the effect of process duration, the etching solution consisting of

1500 ml DI-water, 63 g KOH, and 80 ml IPA was prepared. The etching duration

was up to 45 minutes. The temperatures were 70, 75, and 80oC. Table 1 shows the

parameters of six different as-cut and SDE wafer texturing processes using chemical

solutions based on KOH/IPA/DI-water for magnetic agitation.

58


chemical solutions based on KOH/IPA/DI-water for magnetic agitation.

Process

Name

IPA

(vol%)

KOH

(wt%)

Temperature

(C)

Agitatio

n rate

(rpm)

Process

Time

(min)

70M 5.4 4.2 70 50 15,25, 35,45

75M 5.4 4.2 75 50 15,25,

35,45

80M 5.4 4.2 80 50 15,25,

35,45

Figure 46 shows the reflection measurement results for as-cut wafers. As can

be seen from the figure, as the process time increases, the reflection decreases. The

minimum reflection is obtained for 45 min texturing. When the reflection results at

three different temperatures are compared, the minimum reflection is obtained in

process 75M. Therefore, the optimum temperature for these concentrations is 75o C

for as-cut wafers being textured with magnetic agitation.

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

70

80

90

Wavelength (nm)

non-textured

non-textured^2

70M_15min

70M_25min

70M_35min

70M_45min

Re

fle

cti

on

(%

)

(a)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

non-textured

non-textured^2

75M_15min

75M_25min

75M_35min

75M_45min

Re

fle

cti

on

(%

)

Wavelenght (nm)

(b)

Figure 46: Reflection of textured as-cut wafers for process 5.4vol% IPA, 4.2wt% KOH

with magnetic agitation at (a) 70oC (b) 75

oC (c) 80

oC

59

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50 non-textured

non-textured^2

80M_15min

80M_25min

80M_35min

80M_45min

Re

fle

cti

on

(%

)

Wavelength (nm)

(c)

Figure 46: (Continued)

Figure 47 shows SEM images of as-cut wafers for process 5.4vol% IPA,

4.2wt% KOH at 75oC. As can be seen from the images, as the texturing time

increases, area with pyramids increases which leads to lower reflection. The pyramid

heights can be identify clearly, and they are between 1.5 -11 µm for 15 min

texturing, 3 -16 µm for 25 min texturing, 9 -20 µm for 35 min texturing. After 35

min texturing, the bases of pyramids nest and it makes difficult to identify the heights

correctly. It can be said that as the process duration increases, the pyramid heights

also increase.

60



Figure 48 shows the reflection measurement results for SDE wafers. The

results show the same trend as that of as-cut wafers. As the process duration

increases, reflection decreases. Again the minimum reflection is obtained by 75o C

45 min texturing and it is almost equal to reflection values of as-cut wafer with

respective wavelengths.

61

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

70

80

Wavelength (nm)

non-textured

non-textured^2

70M_15min

70M_25min

70M_35min

70M_45min

Re

fle

cti

on

(%

)

(a)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

70

80

Wavelength (nm)

non-textured

non-textured^2

75M_15min

75M_25min

75M_35min

75M_45min

Re

fle

cti

on

(%

)

(b)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

70

Wavelength (nm)

non-textured

non-textured^2

80M_15min

80M_25min

80M_35min

80M_45min

Re

fle

cti

on

(%

)

(c)

Figure 48: Reflection of textured SDE wafers for process 5.4vol% IPA, 4.2wt% KOH

with magnetic agitation at (a) 70o C (b) 75

o C (c) 80

o C

Figure 49 shows the SEM images of SDE wafers textured at 75o C. Again the

area without pyramid decreases by increasing process duration. The pyramid heights

are 3 -10 µm for 15 min texturing, 4.5-16 µm for 25 min texturing, 5 -20 µm for 35

min texturing. The pyramid heights also increase with process duration.

62

Figure 49: SEM images of textured SDE wafers with magnetic agitation with a solution

consisting of 5.4vol% IPA, 4.2wt% KOH for 15, 25, 35 and 45 min at 75 C

5.1.2 Effect of KOH Concentration

To see the effect of KOH concentration, wafers were textured by using

concentrations given in Table 2. In these processes, KOH concentration was

decreased to 3.33 wt%, and the other parameters were the same as indicated in Table

1.


chemical solutions based on KOH/IPA/DI-water for magnetic agitation.

Process

Name

IPA

(vol%)

KOH

(wt%)

Temperature

(C)

Agitation

rate

(rpm)

Process

Time

(min)

70K 5.4 3.33 70 50 15,25, 35,45

75K 5.4 3.33 75 50 15,25,

35,45

80K 5.4 3.33 80 50 15,25,

35,45

63

Figure 50 shows the reflection measurements of the as-cut wafers. At 70o C, it

can be seen that the reflection of 70K process is lower than that of 70M process for

all process durations when the Figure 48 (a) and Figure 50 (a) are compared.

At 75o C, when the Figure 48 (b) and Figure 50 (b) are compared, reflection for 75K

is lower than that of 75 M process for 15, 25, and 35 min process durations. The

reflection value for 75K is larger than that of 75M for 45 min process duration, but

the difference is almost 1%. At 80o

C, the situation is the same as 70K. Therefore,

decreasing the KOH concentration to 3.33wt % makes the magnetic agitation more

effective for shorter process durations than 45 min.

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

Wavelength (nm)

non-textured

non-textured^2

70K_15min

70K_25min

70K_35min

70K_45min

Re

fle

cti

on

(%

)

(a)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

Wavelength (nm)

non-textured

non-textured^2

75K_15min

75K_25min

75K_35min

75K_45minR

efl

ec

tio

n (

%)

(b)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

Wavelength (nm)

non-textured

non-textured^2

80K_15min

80K_25min

80K_35min

80K_45min

Re

fle

cti

on

(%

)

(c)



o C (c) 80

o C

64

Figure 51 shows the SEM images for process 75K. When pyramids heights

are compared, 75K pyramid heights are smaller than that of 75M. As process

duration increases, area without pyramids decreases as shown in Figure 52 showing

area covered by pyramids. From Figure 52 , the area filled by pyramids for 75K

process is larger than that of 75M for 15 min, 25 min and 35 min process durations.



65

15 20 25 30 35 40 45

20

40

60

80

100

Are

a c

ov

ere

d b

y p

yra

mid

s (

%)

Process Duration (min)

75M_Ascut

75K_Ascut

Figure 52: Area covered by pyramids with respect to time and KOH concentration for

AS-CUT wafers at 75oC

Figure 53 shows the images of 80M and 80K for 45 min process duration. As

can be seen from the figure, although the wafer textured with 4.2wt% KOH is not

covered with pyramids fully, the whole surface of the wafer textured with 3.33wt%

KOH is covered. From the SEM images, it can be concluded that increasing process

duration or temperature is not enough to get fully covered surface, i.e. all

components of the etch solution must be optimized.

Figure 53: SEM images of samples (a) textured with 5.4wt% KOH (b) textured with

3.33wt% KOH at 80oC

66

Figure 54 shows the reflection measurements for SDE wafers. When the

results are compared, the wafers textured with 4.2wt% KOH have again larger

reflection values than the wafers textured with 3.33wt% KOH up to 10% for 15 min,

25 min, and 35 min process durations. For 45 min process duration, the differences

are about 2%, 1%, and 5% at 70, 75, and 80oC respectively.

When Figure 50 and Figure 53 are compared, the reflection for as-cut and

SDE wafers are different from each other about %2 in average at same process

temperature and same process time. It showed that the process with these parameters

was less sensitive to surface morphology than the process parameters with 4.2wt%

KOH.

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

Wavelength (nm)

non-textured

non-textured^2

70K_15min

70K_25min

70K_35min

70K_45min

Re

fle

cti

on

(%

)

(a)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

Wavelength (nm)

non-textured

non-textured^2

75K_15min

75K_25min

75K_35min

75K_45min

Re

fle

cti

on

(%

)

(b)

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

Wavelength (nm)

non-textured

non-textured^2

80K_15min

80K_25min

80K_35min

80K_45min

Re

fle

cti

on

(%

)

(c)

Figure 54: : Reflection of textured SDE wafers for process 5.4vol% IPA, 3.33wt%


oC (c) 80

oC

67

Figure 55 and Figure 56 show the SEM images of SDE wafers textured at

75oC and the graph of covered area with respect to time respectively. The wafers

textured with 3.33wt % KOH have larger area with pyramids than wafers textured

with 4.2wt % KOH have for process durations up to 35 min as can be seen from the

figures. For 45 min process time, both are covered fully with pyramids.

Figure 55: SEM images of textured SDE wafers with magnetic agitation with a solution

consisting of 5.4vol% IPA, 3.33wt% KOH for 15, 25, 35 and 45 min at 75oC

15 20 25 30 35 40 45

20

40

60

80

100

4.2wt% KOH

3.3wt% KOH

Are

a c

ov

ere

d b

y p

yra

mid

s (

%)


Figure 56: Area covered by pyramids with respect to time and KOH concentration for

SDE wafers at 75oC(Lines are just added to guide the eye)

68

It is realized that both SDE and as-cut wafers textured with magnetic

agitation with a solution consisting of 5.4vol% IPA, 3.33wt% KOH have low

reflection and large coverage with pyramids for process durations 15 min, 25 min,

and 35 min. Figure 57 shows the area covered by pyramids with respect to different

process durations and temperatures. As can be seen from the Figure 57, process does

not depend on surface morphology very much for the solution with 3.3wt% KOH

and the minimum coverage is about 80% for 15min process duration. For the

following experiments, KOH concentration was kept constant as 3.33wt%, and the

other parameters were changed to get full covered wafers with pyramids.

Figure 57: Area covered by pyramids of textured wafers with magnetic agitation with a

solution consisting of 5.4vol% IPA, 3.33wt% KOH for 15, 25, 35 and 45 min at (a) 70oC

(b) 75oC (c) 80

oC

69

5.1.3 Effect of using a cap to Keep IPA Constant

Closing the beaker increases the pressure on the solution, and the evaporation

of water and IPA decreases. This makes the concentrations almost constant during

process. To see this effect on process, wafers were textured with parameters given in

Table 3.

Table 3: Parameters of two different AS-CUT/SDE wafer texturing processes using

chemical solutions to observe effect of using a cap

Process

Name

IPA

(vol%)

KOH

(wt%)

Temperature

(C)

Agitation

rate (rpm)

Process Time

(min)

75MC 5.4 4.2 75 50 25,45

70KC 5.4 3.33 70 50 15,25

Figure 58, Figure 59 and Figure 60 show the reflection results, SEM images

of as-cut wafers and SEM images of SDE wafers for processes 75M and 75MC. As

can be seen from Figure 58, reflection decreases dramatically for as-cut and SDE

wafers, but they are still high. Figure 59 and Figure 60 indicate that coverage with

pyramids increases from 21% to 92% for as-cut wafer and from 35% to 82% for SDE

wafer. The results show that using cap can decrease reflection or increases coverage

for process 75M_25min.

300 400 500 600 700 800 900 1000 1100 120010

15

20

25

30

35

40

45

50

55

60

Re

fle

cti

on

(%

)

Wavelength (nm)

75M_ascut

75MC_ascut

75M_sde

75MC_sde


4.2wt % KOH, 5.4vol % IPA, 25 min at 75oC with and without a cap.

70

Figure 59: SEM images of as-cut wafers textured with a solution consisting of 5.4vol %

IPA, 4.2wt % KOH for 25 min process duration at 75oC a) without cap (coverage 21%)

b) with cap (coverage 92%)

Figure 60: SEM images of SDE wafers textured with a solution consisting of 5.4vol %

IPA, 4.2wt % KOH for 25 min process duration at 75oC a) without cap (coverage 35%)

b) with cap (coverage 82%)

Figure 61 and Figure 62 show reflection measurements and SEM images of

wafers with a solution consisting of 4.2wt % KOH, 5.4vol % IPA, 45 min at 75 C

with and without a cap. From Figure 61, using a cap decreases the AM 1.5 weighted

reflection about 0.5% for as-cut and 0.8% for SDE wafers. When the images shown

in Figure 62 are analyzed, the average pyramid heights and density are 6.74 µm and

1.12x104 mm

-2 for as-cut, 7.5 µm and 0.889x10

4 mm

-2 for SDE wafers textured

71

without using a cap. Using a cap decreases the pyramid height to 2.38 µm for SDE

and to 2.19 µm for as-cut, and increases pyramid density to 0.883x105 mm

-2 for SDE

and to 1.04x105 mm

-2 for as-cut.

500 600 700 800 900 10009

10

11

12

13

14

15

Re

fle

cti

on

(%

)

Wavelenght (nm)

75M_45min_ascut

75MC_45min_ascut

12.9%12.4%

(a)

500 600 700 800 900 10008

9

10

11

12

13

14

15

Wavelength (nm)

Re

fle

cti

on

(%

)

75M_45min_SDE

75MC_45min_SDE

12.9%

12.1%

(b)


4.2wt % KOH, 5.4vol % IPA, 45 min at 75oC with and without cap a) as-cut b) SDE

Figure 62: SEM images of wafers textured with a solution consisting of 5.4vol % IPA,

4.2wt % KOH for 45 min process duration at 75oC a) as-cut b) SDE with and without

using a cap

72

Figure 63 shows the reflection measurements of wafers textured with a

solution consisting of 3.33wt % KOH, 5.4vol % IPA, 15min and 25 min at 70 C with

and without cap. Using a cap increases the reflection very much. Therefore, SEM

images of this process were not taken.

These results show that only using a cap is not enough to get fully covered

surfaces for short process durations (i.e. like 15 min and 25 min), and small pyramids

shows lower reflection than big ones although the difference is not very much. After

that point,effects of increasing the speed of magnetic stirring were observed.

300 400 500 600 700 800 900 1000 1100 120010

15

20

25

30

35

40

45

50

55

60

65

70KC_15min_ascut

70KC_25min_ascut

70K_15 min_ascut

70K_25 min_ascut

Wavelength (nm)

Re

fle

cti

on

(%

)

(a)

300 400 500 600 700 800 900 1000 1100 1200

15

20

25

30

35

40

45

50

55

Re

fle

cti

on

(%)

Wavelength (nm)

70KC_15min_sde

70KC_25min_sde

70K_15 min_sde

70K_25 min_sde

(b)


3.33wt % KOH, 5.4vol % IPA, 15min and 25 min at 70oC with and without cap a) as-

cut b) SDE

5.1.4 Effect of Magnetic Stirring Speed

To see the effect of magnetic stirring speed, wafers were textured with

parameters given in Table 4. These parameters were chosen because previous

process 70KC whose parameters given in Table 3 gives high reflection values. The

aim was to see whether there would be a recovery in texturing or not.

73

Table 4: Parameters of the solutions to observe the effect of the magnetic stirring speed

Process

Name

IPA

(vol%)/ ml

KOH

(wt%)

Temperature

(C)

Agitation

rate (rpm)

Process Time

(min)-Wafer

Surface

70KCS 5.4 3.33 70 125 15,25-

ASCUT

25,35-SDE

75KCS 5.4 3.33 70 125 25,35-SDE

Figure 64 shows the reflection results for process 70KCS as-cut wafers.

When the agitation speed was increased to 125 rpm, the reflection decreased. The

reflection and the coverage are related as the reflection decreases, coverage

increases, so coverage also increases with increasing speed. But the reflection value

was higher than the desired one about 10% for 15min and 5% for 25min and

wavelengths between 550 and 1000 nm. Therefore I did not put the SEM images of

these wafers.

300 400 500 600 700 800 900 1000 1100 1200

15

20

25

30

35

40

45

50

55

60

65

Re

fle

cti

on

(%

)

Wavelength (nm)

70KC_15min

70KC_25min

70KCS_15min

70KCS_25min

Figure 64: Reflection measurements for as-cut wafers textured with a solution

consisting of 3.33wt % KOH, 5.4vol % IPA, 15min and 25 min at 70oC with a speed of

50 and 125 rpm

74

Figure 65 shows the reflection measurements for SDE wafers textured with

solution parameters given above. At 70oC, SDE wafers showed high reflection values

for process durations 25min and 35 min, although the process was carried out by

using cap and with a high agitation speed. Process was repeated at 75oC with a high

agitation speed, and the reflection values decreased to sublimit which is square of

reflection of non-texture SDE wafer. Therefore, to get fully textured SDE wafers at

short process time with using a cap, minimum temperature should be 75oC.

300 400 500 600 700 800 900 1000 1100 1200

10

15

20

25

30

35

40

45

50

70KCS_25min

70KCS_35min

75KCS_25min

75KCS_35min

70K_25min

70K_35min

non-texture^2

Re

fle

cti

on

(%

)

Wavelength (nm)

Figure 65: Reflection measurements for SDE wafers textured with a solution consisting

of 3.33wt % KOH, 5.4vol % IPA, 25min and 35 min at 70 and 75oC with a speed of 50

rmp and 125 rpm

5.1.5 Effect of IPA Concentration

To see the effect of IPA concentration, wafers were textured with parameters

given in Table 5. In this experiment, the parameters were 70oC, 3.33wt % KOH, 125

rpm agitation speed, process durations 15 min and 25 min like process parameters of

70K. The differences from 70K were using a cap and the agitation speed. The IPA

was added in three different volume 40 ml, 80 ml, and 120 ml.

75


chemical solutions to observe effect of IPA concentration

Process

Name

IPA

(vol%)/

(ml)

KOH

(wt%)

Temperature

(oC)

Agitation

rate (rpm)

Process

Time

(min)

1I 2.67/40 3.33 70 125 15,25

2I 5.4/80 3.33 70 125 15,25

3I 8/120 3.33 70 125 15,25

Figure 66 shows the reflection measurements of wafers textured with

parameters given in Table 5. As can be seen from the figure, as the IPA

concentration increases, reflection also increases for 15 min and 25 min process

durations. In other words, area with pyramids decreases. Figure 67 shows mass loss

of wafers. It can be seen from the figure that increasing IPA decreases the etch rate

and mass loss with respective process durations.

300 400 500 600 700 800 900 1000 1100 120020

25

30

35

40

45

50

55

60

40ml IPA

80ml IPA

120ml IPA

R

efl

ec

tio

n (

%)

Wavelength (nm)

(a)

300 400 500 600 700 800 900 1000 1100 1200

15

20

25

30

35

40

45

40ml IPA

80ml IPA

120ml IPA

Re

fle

cti

on

(%

)

Wavelenght (nm)

(b)

Figure 66: Reflection values of as-cut wafers wafers textured with a solution consisting

of 3.33wt % KOH, different IPA concentrations a) 15min and b) 25 min at 70oC with a

speed of 125 rpm

76

15 256

8

10

12

14

16

18

20

Ma

ss

lo

ss

(%

)

Time (min)

40ml IPA

80ml IPA

120ml IPA

Figure 67: Mass loss with respect to time and changing IPA concentration (Lines are

just added to guide the eyes)

Although minimum reflection values were obtained by using 40ml IPA, the

following experiments were not done with this amount. We observed that the surface

of the wafer after texturing process has some features as shown in Figure 68. This

may be due to low IPA concentration causing less wettability of the surface, and H2

bubbles generated during the etching process may not leave the surface as quickly as

possible. That is why these features are not seen the wafers textured with a solution

consisting of 80 ml and 120 ml IPA. Therefore, the concentration of IPA was kept as

5.4 vol% for other experiments.

Figure 68: Textured wafer with a solution consisting of 40ml IPA, 3.33wt % KOH, 15

min at 70oC

77

5.2 Texturing Process with Ultrasonic Agitation

To observe the ultrasonic effect, the caustic etching solutions were prepared

as 3.33 wt% KOH, 5.4 vol% IPA at 60oC and 70

oC for process durations 15 min, 25

min, 35min, and 45 min.

Firstly, 5x5 cm2, p-type, 1-3 Ω-cm, (100) CZ-Si wafers wafers were textured.

The solution was prepared in the glass beaker shown in Figure 33 and stirred by

using magnetic agitation and heated up to 70oC. The heated solution was put in big

ultrasonic cleaner which was filled by water and heated to 70oC to make the solution

temperature stable. The as-cut wafers were textured for process durations 15min and

25 min like process 70K or 70KC.

Figure 69 shows a) reflection values, b) and c) AM 1.5-weighted reflection

(AM 1.5-WR) values. When the reflection values and AM 1.5-WR values are

compared, there is a dramatic fall in the reflection of textured wafers for process

durations 15min and 25 min about 5%, 10% and 20% when compared with process

without cap, process 25min , and process 15min with cap respectively, as seen in

Figure 69 (a). In Figure 69 (c), the AM 1.5-WR values are almost same except for

processes 70M_45min and 80M_45min which were not textured well. Therefore, the

ultrasonic agitation provided low reflection values, which were obtained for process

duration 45min at process temperature at 75oC or 80

oC with magnetic agitation, for

process durations 15min and 25min at 70oC.

78

300 400 500 600 700 800 900 1000 1100 1200

10

20

30

40

50

60

R

efl

ec

tio

n (

%)

Wavelength (nm)

MA with cap_15 min

MA with cap_25 min

MA without cap_15 min

MA without cap_25 min

UA_15 min

UA_15 min

non-texture^2

(a)

14 16 18 20 22 24 26

10

15

20

25

30

35

40

AM

1.5

-we

igh

ted

re

fle

cti

on

(%

)

Time (min)

non-textured^2

MA with cap

MA without cap

UA

(b)

10

12

14

16

18

20

22

A

M 1

.5-w

eig

hte

d r

efl

ec

tio

n (

%)

UA_15min

UA_25min

75M_45min

80M_45min

70M_45 min

75K_35min

70K_45min

80K_45min

(c)

Figure 69: (a) Reflection values for textured as-cut wafers with magnetic agitation

(MA) and ultrasonic agitation (UA) (b) AM 1.5-weighted reflection (AM 1.5-WR)

values (c) Lowest reflection values obtained up to now(Lines are just added to guide the

eyes)

The surface of as-cut wafers textured with ultrasonic agitation was fully

covered by pyramids for process durations 15min, and 25min. Therefore the SEM

pictures were not put here. Figure 70 shows the SEM images in cross sectional view

of wafers whose reflection values are lowest ones. As can be seen, the pyramid

heights are up to 17 µm for wafers textured with magnetic agitation, and the pyramid

heights are up to 4 µm for wafers textured with ultrasonic agitation. Therefore, the

79

pyramid density of wafers textured with ultrasonic agitation is larger about 4 times

than that of wafers textured with magnetic agitation.

Figure 70: SEM images of a) 75M_45min as-cut wafer b) 80K_45min SDE wafer

c) UA_15min as-cut wafer

After that point, ultrasonic texturing processes for process durations 15min

and 25min were repeated, and reproducibility of the process was provided. Then,

15.6x15.6 cm2, p-type, 1-3 Ω-cm, (100) CZ-Si, 25 wafers for each process durations

were textured with parameters given in Table 6, by using a cassette. These wafers

were used for solar cell production.

Table 6: Parameters of two different AS-CUT wafer texturing processes using chemical

solutions

Process

Name

IPA

(vol%)/ ml

KOH

(wt%) /

g

Temperature

(oC)

Agitation Process

Time (min)

70UA 5.4/ 427 3.33/266 70 Ultrasonic 15,25,35,45

60UA 5.4/427 3.33/266 60 Ultrasonic 15,25,35,45

80

Figure 71 shows the reflection measurements and AM 1.5 weighted ones. At

60oC, reflection values decreases sharply from 15 min to 25 min, and 25 min to 35

min. But reflection values decrease slightly at 70oC as texturing time increases.

300 400 500 600 700 800 900 1000 1100 12005

10

15

20

25

30

15 min

25 min

35 min

45 min

non-textured^2

Re

fle

cti

on

(%

)

Wavelength (nm)

(a)

300 400 500 600 700 800 900 1000 1100 12005

10

15

20

25

30

35

40

Re

fle

cti

on

(%

)

Wavelength (nm)

15 min

25 min

35 min

45 min

non-textured^2

(b)

15 20 25 30 35 40 45

12

14

16

18

20

22

24

AM

1.5

We

igh

ted

Re

f le

cti

on

(%

)

Time (min)

70 C

60 C

(c)

Figure 71: Reflection measurements of as-cut wafers textured by ultrasonic agitation at

a) 70oC b) 60

oC c) AM 1.5 weighted reflection values (Lines are just added to guide the

eyes)

Figure 72 a) shows the surface of wafer textured for 15 min at 70oC. The

coverage is inhomogeneous with small pyramids between 0.8-1.5 µm and large

pyramids between 3-6µm. Although surface is covered fully, the reflection values is

high (see Figure 71) because the pyramids have round tips and corners.

81

Figure 72 a) shows the SEM image of wafer textured for 25 min at 70oC. The

coverage almost homogenous with pyramids heights between 4-7 µm. Due to the

homogeneity of pyramids and completed pyramids, the reflection value lower than

that of wafer textured for 15 min about 1% (see Figure 71)

Figure 72 a) shows the SEM images of wafer textured for 35 min and 45 min

at 70oC. The distribution of pyramids is almost same and inhomogeneous. There are

almost same in amount of large pyramids (4-6 µm) and small pyramids (1.5-3

µm).The reflection values almost same as seen in Figure 71.

Figure 72 b) shows the SEM images of wafer textured for 15min, 25min,

35min and 45min at 60oC. For 15min and 25 min, the surface is covered with

pyramids but some of them are not completed, and this makes reflection values

higher than that of wafers textured at 70oC (see Figure 71). But the coverage of 25

min process is higher than that of 15 min process. For 35 min and 45 min, surface is

fully covered with pyramids. The reflection values are almost same.

(a)

Figure 72: SEM images of as-cut wafers textured by ultrasonic agitation at a) 70oC b)

60oC

82

(b)


Figure 73 shows a) the average pyramid height, b) pyramid density with

respect to time and temperature. The average pyramid heights or pyramid densities of

wafers textured for 15 min and 25 min at 60oC are not added because the surface of

them is not fully covered (see Figure 72(b)). The pyramid height of 25 min process

duration is the highest value at 70oC. While the density increases from 35 min to 45

min process time at 70oC, this value decreases at 60

oC.

Figure 73 c) shows the mass loss with respect to time at both process

temperatures. The mass loss sharply increases from 15 min to 25 min process

durations at 70oC and 60

oC. After that point, the amount of etched wafer increases

again but the increment of the mass loss decreases as 2% from 25 min to 35 min,

0.6% from 35min to 45 min at 70oC. It shows that number of (111) planes increases

and the etch rate decreases. The mass loss at 60oC is lower than the mass loss at

70oC about 2%, and the reflection values of wafers textured for 35 min and 45 min at

70 and 60

oC are almost same (see Figure 71 ). In other words, the lowest reflection

83

values can be obtained by etching low amount of wafer (about 7%) at 60oC for 35

min process duration.

15 20 25 30 35 40 450.0

1.0x10-2

2.0x10-2

3.0x10-2

4.0x10-2

5.0x10-2

6.0x10-2

7.0x10-2

8.0x10-2

Py

ram

id D

en

sit

y (m

2)

Time (min)

70o C

60o C

(a)

15 20 25 30 35 40 45

2.5

3.0

3.5

4.0

4.5

Av

era

ge

Py

ram

id H

eig

ht

(m

)

Time (min)

70oC

60oC

(b)

10 20 30 40 503

4

5

6

7

8

9

10

11

Ma

ss

Lo

ss

(%

)

Process duration (min)

70oC

60oC

(c)

Figure 73: a) Pyramid density b) Average Pyramid height c) Mass loss for as-cut

wafers textured with ultrasonic agitation at 70 and 60oC(Lines are just added to guide

the eyes)

5.3 Simulation of Light Trapping

To simulate Si wafer light trapping, we used an optical system design (OSD)

program (Zemax) and a three dimensional solid element modeling program

(Solidworks).

We modeled the wafer textured and non- textured, and put it into the OSD

program. Before making simulations, the absorption and refractive index of mono-Si

data is implemented to produce a glass type acting like mono-Si into the OSD

84

program. The data is taken from PVCDROM and Table 7 shows these values from

250 nm to 1100 nm [41].

Table 7: Absorption coefficient and refractive index values of mono-Si

Wavelength (nm)

Absorption Coefficient (cm

-1)

n (Refractive

Index) Wavelength (nm)

Absorption Coefficient (cm

-1)

n (Refractive

Index)

350 1.04E+06 5.483 730 1.54E+03 3.741

360 1.02E+06 6.014 740 1.42E+03 3.732

370 6.97E+05 6.863 750 1.30E+03 3.723

380 2.93E+05 6.548 760 1.19E+03 3.714

390 1.50E+05 5.976 770 1.10E+03 3.705

400 9.52E+04 5.587 780 1.01E+03 3.696

410 6.74E+04 5.305 790 9.28E+02 3.688

420 5.00E+04 5.091 800 8.50E+02 3.681

430 3.92E+04 4.925 810 7.75E+02 3.674

440 3.11E+04 4.793 820 7.07E+02 3.668

450 2.55E+04 4.676 830 6.47E+02 3.662

460 2.10E+04 4.577 840 5.91E+02 3.656

470 1.72E+04 4.491 850 5.35E+02 3.65

480 1.48E+04 4.416 860 4.80E+02 3.644

490 1.27E+04 4.348 870 4.32E+02 3.638

500 1.11E+04 4.293 880 3.83E+02 3.632

510 9.70E+03 4.239 890 3.43E+02 3.626

520 8.80E+03 4.192 900 3.06E+02 3.62

530 7.85E+03 4.15 910 2.72E+02 3.614

540 7.05E+03 4.11 920 2.40E+02 3.608

550 6.39E+03 4.077 930 2.10E+02 3.602

560 5.78E+03 4.044 940 1.83E+02 3.597

570 5.32E+03 4.015 950 1.57E+02 3.592

580 4.88E+03 3.986 960 1.34E+02 3.587

590 4.49E+03 3.962 970 1.14E+02 3.582

600 4.14E+03 3.939 980 9.59E+01 3.578

610 3.81E+03 3.916 990 7.92E+01 3.574

620 3.52E+03 3.895 1000 6.40E+01 3.57

630 3.27E+03 3.879 1010 5.11E+01 3.566

640 3.04E+03 3.861 1020 3.99E+01 3.563

650 2.81E+03 3.844 1030 3.02E+01 3.56

660 2.58E+03 3.83 1040 2.26E+01 3.557

660 2.58E+03 3.83 1050 1.63E+01 3.554

670 2.38E+03 3.815 1060 1.11E+01 3.551

680 2.21E+03 3.8 1070 8.00E+00 3.548

690 2.05E+03 3.787 1080 6.20E+00 3.546

700 1.90E+03 3.774 1090 4.70E+00 3.544

710 1.77E+03 3.762 1100 3.50E+00 3.541

720 1.66E+03 3.751

85

5.3.1 Simulation of SDE wafer

Figure 74 shows the model of simulation. In this model, there are two

rectangle detectors in front and back of the wafer, one rectangle source and a SDE

wafer. By using this model, reflection detected light intensity by front detector,

transmission detected light intensity by back detector, and absorption, which is

obtained by subtracting addition of reflection and transmission from one hundred,

can be obtained. The wafer thickness is taken as 160 µm because wafers are etched

about 10 µm per side to remove the saw damages, and the wafers used in our

laboratory have thickness about 180 µm.

Figure 74: Model of SDE wafer reflection simulation

Figure 75 a) shows the measured reflection value and simulation result of

SDE wafer. As can be seen, the reflection values of simulation is different from

measured ones about 9% for 350 nm and 375 nm, about 2% between 400 nm and

1025 nm, and the difference is about 4% between 1050 nm and 1100 nm. Our

measurements have an error about 1 %, so the values of simulation obtained between

400nm and 1025 nm differs about 1% from measured reflection values.

Figure 75 shows a) reflection, b) transmission, and c) absorption of light for

wafer thickness of 2, 4, 8, 20, 100, and 180 µm. The reflection, transmission and

absorption values of 350 nm and 500 nm are constant as the thickness increases. It

86

means that for these wavelengths wafer thickness equal and higher than 2 µm is

enough for absorption of light entering the wafer.

300 400 500 600 700 800 900 1000 110030

35

40

45

50

55

60

Experimental

Simulation

Re

fle

cti

on

(%

)

Wavelength (nm )

37.93

37.17

(a)

1 2 4 8 16 32 64 128 25630

35

40

45

50

Re

fle

cti

on

(%

)Wafer thickness (m]

350 nm

500 nm

700 nm

825 nm

1050 nm

1100 nm

(b)

1 2 4 8 16 32 64 128 256

0

10

20

30

40

50

Tra

ns

mis

ion

(%

)

Wafer thickness m)

350 nm

500 nm

700 nm

825 nm

1050 nm

1100 nm

(c)

1 2 4 8 16 32 64 128 256

0

10

20

30

40

50

60

70

Ab

so

rpti

on

(%

)

Wafer thickness (m)

350 nm

500 nm

700 nm

825 nm

1050 nm

1100 nm

(d)

Figure 75: a) Reflection data b) Reflection c) Transmission d) Absorption with respect

to changing thickness (Lines are just added to guide the eyes)

For wavelengths 700 nm and so on, the reflection values decreases as the

thickness increases. It shows that reflection is not due to front reflection, but also due

to light reflected from back surface of the wafer. To obtain zero transmission, the

thickness must be at least 20 µm for 700 nm, 100 µm for 825 nm, and the thickness

must be higher than 2 mm and 8 mm, which are not shown, for 1050 and 1100 nm

respectively.

5.3.2 Simulation of Wafer with Uniformly Distributed Pyramids

First of all, the aim was to simulate random pyramids but we faced with a

problem. The problem is that when one pyramid is put on a rectangle volume, the

87

OSD program perceives as there are a pyramid, air, and a rectangle volume.

Therefore, light passing through the pyramid is reflected back without entering the

rectangle volume. The situation is illustrated in Figure 76 (a). Figure 76 (b) shows

the working model.

(a)

(b)

Figure 76: Simulation of textured wafer obtained by a) assembling one pyramid on a

rectangle volume b) cutting the a part of rectangle volume

The problem was not solved. Therefore, we modeled a wafer surface with

uniformly distributed pyramids as shown in Figure 77 (a). The simulation model is

shown in Figure 77 (b). Here, there is a uniformly textured wafer model instead of

SDE wafer model (see Figure 74) .

(a)

(b)

Figure 77: a) Uniformly textured wafer model b) Simulation model

In the simulations, the total thickness is kept as constant, and it is equal to

sum of heights of front and back pyramids, bare wafer thickness as shown in Figure

77 (b). Figure 78 shows reflection values of reference wafer (randomly textured with

88

average pyramid height 7.5 µm), and model with 1, 6, 10 µm pyramids. From the

figure, the AM 1.5-weighted reflection values of models are higher than reference

about 1%. Although Zemax does calculations by using geometric optic, there are

small differences for wavelength higher than 1050 nm as shown in figure. This is due

to the fluctuations in reflected light from back surface of the wafer for these

wavelengths.

300 400 500 600 700 800 900 1000 11008

12

16

20

24

28

Model with 1m pyramids



Referans measured

Re

fle

cti

on

(%

)

Wavelength (nm)

(a)

0

2

4

6

8

10

12

14




Referans measured

AM

1.5

-We

igh

ted

Re

fle

cit

on

(%

)

(b)

Figure 78: a)Reflection values for model with 1,6,10 µm pyramids b) AM 1.5-Weighted

Reflection

In addition, two side textured wafers with 6 µm pyramid heights and different

total thicknesses were simulated. The absorption result is shown in Figure 79. From

the figure, the absorption decrease with decrease in thickness between 925 nm and

1100 nm, and the absorption values are same between 350 nm and 925 nm.

89

800 850 900 950 1000 1050 1100

20

30

40

50

60

70

80

90

100 m thick

140 m thick

180 m thickAb

so

rpti

on

(%

)

Wavelength (nm)

Figure 79: Absorption graph of two side textured wafer with 6 µm pyramid height

We also simulated one side textured wafers with different total thicknesses

and different pyramid heights. Thicknesses higher than 180 µm were not taken,

because it will be not cost effective and nowadays trend is production of solar cells

with thin wafers.

Table 8: Parameters of models

Model Name Total Thickness (µm) Pyramid Heights (µm)

100_1side 100 1-6-10

140_1sdie 140 1-6-10

180_1side 180 1-6-10

Figure 80 shows the absorption values for wafers with different pyramid sizes

and thicknesses. The wavelength is taken from 950nm to 1100nm, because the

absorption values almost same up to 950 nm.

In Figure 80 (a), one side textured wafers have higher absorption than two

side textured. Because total reflection probability of light passing through the wafer

and facing with flat surface is higher than that of light facing with surface with

pyramids as shown in Figure 77 (b) Figure 81.

90

As can be seen from Figure 80 (b)- (c)- (d), increasing thickness increases the

absorption for the same pyramid sizes. When thickness is constant and the pyramid

sizes are changed, the absorption values increases with increasing pyramid sizes after

1025 nm.

950 1000 1050 110015

30

45

60

75

90

180_1side_1m pyramids






Ab

so

rpti

on

(%

)

Wavelength (nm)

(a)

950 975 1000 1025 1050 1075 110020

30

40

50

60

70

80

90

Ab

so

rpti

on

(%

)

Wavelength (nm)

180_1side_1 m pyramids



(b)

950 975 1000 1025 1050 1075 110020

30

40

50

60

70

80

90

Ab

so

rpti

on

(%

)

Wavelength (nm)




(c)

950 975 1000 1025 1050 1075 1100

30

40

50

60

70

80

90

Ab

so

rpti

on

(%

)

Wavelength (nm)




(d)

Figure 80: Absorption of a) one side and two side textured wafer, b)one side wafer with

1 µm pyramids, c) one side wafer with 6 µm pyramids d) one side wafer with 10 µm

pyramids

Figure 81: Model of a wafer with one side texturing

91

5.4 Solar Cell Results

Screen printed Cz silicon solar cells with dimensions 15.6x15.6 cm2 were

produced. The wafers of cells were 70UA and 60UA whose parameters were given

in Table 9. For each texture parameter, 10 wafers were used (total 90 wafers) to

produce solar cells. To get optimum SiNx layer, each set was divided into groups

with 5 wafers, and average values were taken from one of these groups. The

reference cell was the standard cell produced in GÜNAM laboratory.

Table 9: I-V measurement results by Solar Simulator

Texture

Temperature

(oC)

Time of

texturing

(min)

Average

Jsc

(mA)

Average

Voc

(V)

Average

FF

(%)

Average

Efficiency

(%)

Average

Series

(mΩ)

70UA

70 15 34.612 0.623 79.715 17.204 6.91

70 25 34.648 0.623 79.717 17.218 6.89

70 35 35.686 0.623 78.996 17.587 7.17

70 45 35.687 0.623 79.051 17.601 7.16

60UA

60 15 33.651 0.62 79.535 16.734 7.14

60 25 34.71 0.625 79.546 17.264 7.01

60 35 34.453 0.622 79.744 17.235 6.91

60 45 35.617 0.623 79.314 17.608 6.94

Reference 75 45 34.434 0.619 79.659 16.989 6.84

Table 9 shows the I-V measurement results by Solar Simulator. Figure 82

shows the related curves. Average Jsc increases as process duration increases at

different temperatures. And the Jsc of reference is lower than the average Jsc of the

cells textured with ultrasonic agitation. In Figure 82 (b), reference has the lowest Voc

and the cells textured for 10 min at 70oC have the highest value about 626 mV. The

others have almost same average Voc values about 623 mV except for 15 min

texturing at 60oC. In Figure 82 (c), fill factor values are about 79%. Figure 82 (d)

shows all the efficiencies with respect to all Jsc values of 100 cells. As can be seen,

efficiency is almost linearly proportional to Jsc, and the highest efficiency is 17.737%

92

of cell textured for 45min at 60oC and the related I-V curve is shown in Figure 82 (f).

Average cell efficiency of reference is the lowest one when compared with the others

except for cell textured for 15 min at 60oC as shown in the figure (e). Average cell

efficiency increases with increase in process duration at different process

temperature.

15 20 25 30 35 40 45

33.5

34.0

34.5

35.0

35.5

36.0

70o C

60o C

reference

Texturing time (min)

Av

era

ge

Js

c (

mA

/cm

2)

(a)

15 20 25 30 35 40 45

619

620

621

622

623

624

625

626


70o C

60o C

reference

Av

era

ge

Vo

c (

mV

)

(b)

15 20 25 30 35 40 45

78.9

79.2

79.5

79.8

70o C

60o C

referans


Av

era

ge

Fil

l F

ac

tor

(%)

(c)

31 32 33 34 35 36

15.5

16.0

16.5

17.0

17.5

18.0

Jsc (mA/cm2)

Eff

icie

nc

y (

%)

(d)

Figure 83: Graphs of Average a) Jsc b) Voc c) Fill factor e) Efficiency with respect to

texturing time d) Efficiency- Jsc, f) I-V curve of best cell

93

15 20 25 30 35 40 4516.6

16.8

17.0

17.2

17.4

17.6

17.8


70o C

60o C

referenceA

ve

rag

e E

ffic

ien

cy

(%

)

(e)

0 100 200 300 400 500 600 7000

5

10

15

20

25

30

35

40

- Jsc= 35.85 mA cm-2 / 34.43 mA cm

-2

- Voc= 0.623 mV / 0.619 mV

- FF= 79.43 % / 79.718 %

- Eff= 17.74% / 17.00 %

Js

c (

mA

cm

-2)

Voltage (mV)

Best Reference

Best Cell

(f)


Figure 83 shows the mass loss and the Jsc values with changing texturing

time. As the mass loss increases with increase in process duration, Jsc increases. One

can expected that Jsc of thick wafer should be higher than that of thin wafer for same

reflection values for wafers without ARC. Because the absorption increases when

thickness is increased (see Figure 79 ). In our case, wafers textured for 35 min and

45 min at 60oC are thicker than wafers textured for 35 min and 45 min at 70

oC. But

the Jsc values are almost same. It is because the mass loss is too low to observe such a

change in Jsc. In Figure 79, mass loss about 22% (i.e. 140 µm thickness) decreases

the absorption about 2%, but in our case maximum mass loss is about 10%.

15 20 25 30 35 40 453

4

5

6

7

8

Mass loss

Average Jsc


Ma

ss

lo

ss

(%

)

33.5

34.0

34.5

35.0

35.5

36.0

Av

era

ge

Js

c (m

A/c

m2

)

(a)

15 20 25 30 35 40 454

5

6

7

8

9

10

11

Mass loss

Average Jsc


Ma

ss

lo

ss

(%

)

34.6

34.8

35.0

35.2

35.4

35.6

35.8

Av

era

ge

Js

c (m

A/c

m2

)

(b)

Figure 83: Mass loss and Jsc with changing process durations a) at 60oC b) at 70

oC

94

Figure 84 shows how the average Jsc changes with the AM 1.5 weighted

reflection of wafers with ARC and texturing time. As the reflection decreases, the Jsc

values increases except for 35 min at 60oC. This set shows also the unexpected value

of efficiency, so there should be a production error for this set.

15 20 25 30 35 40 45

3.0

3.5

4.0

4.5

5.0

5.5

6.0

A.M 1.5

Average Jsc


AM

1.5

-we

igh

ted

re

fle

cti

on

(%

)

33.5

34.0

34.5

35.0

35.5

36.0

Av

era

ge

Js

c (m

A / c

m2)

(a)

15 20 25 30 35 40 45

3.0

3.5

4.0

4.5

5.0 A.M 1.5

Average Jsc


AM

1.5

-we

igh

ted

re

fle

cti

on

(%

)

34.5

35.0

35.5

36.0

Av

era

ge

Js

c (m

A / c

m2)

(b)

Figure 84: AM 1.5-weighted reflection and Average Jsc values with respect to changing

texturing time a) at 60oC b) 70

oC

To see the performance of produced cells without effects of series resistance,

the cell parameters were measured by Suns-Voc system. The measurement results

are shown in Table 10.

Table 10: Suns-Voc measurement results

Texture

Temperature

(oC)

Time of

texturing

(min)

Average

Jsc

(mA)

Average

Voc

(V)

Average

FF

(%)

Average

Eff

(%)

70UA

70 15 34.612 0.615 83.113 17.694

70 25 34.648 0.614 83.087 17.700

70 35 35.686 0.614 83.101 18.222

70 45 35.687 0.615 82.94 18.232

60UA

60 15 33.651 0.614 82.587 17.071

60 25 34.71 0.617 82.891 17.767

60 35 34.453 0.616 82.804 17.582

60 45 35.617 0.614 82.864 18.132

Reference 75 45 34.434 0.609 83.013 17.42

95

Figure 85 shows the pseudo efficiencies and fill factors. Pseudo efficiencies

and fill factors are higher than real ones. Real efficiencies and FF are cell parameters

depending on series resistance, and they decrease with increase in series resistances.

15 20 25 30 35 40 45

16.8

17.0

17.2

17.4

17.6

17.8

18.0

18.2

Eff

icie

nc

y (

%)

Time of texturing (min)

pseudo_60

real_60

(a)

15 20 25 30 35 40 45

17.2

17.3

17.4

17.5

17.6

17.7

17.8

17.9

18.0

18.1

18.2

18.3

Eff

icie

nc

y (

%)


pseudo_70

real_70

(b)

15 20 25 30 35 40 45

79.5

80.0

80.5

81.0

81.5

82.0

82.5

83.0

Av

era

ge

FF

(%

)


pseudo_60

real_60

(c)

15 20 25 30 35 40 45

79.0

79.5

80.0

80.5

81.0

81.5

82.0

82.5

83.0A

ve

rag

e F

F (

%)


pseudo_70

real_70

(d)

Figure 85: Pseudo and real efficiencies and fill factors a),c) at 60oC b),d) at 70

oC

process temperature

As shown in Table 10, the Voc value of reference is lower than the others

about 5 mV. If we use Equation 4 to find the saturation currents and then use the Isc

of 70UA_45 min to find reference cell Voc, we see that the calculated Voc of

reference increases about 0.9 mV which is much smaller than 4 mV. It shows that

increase in Voc is not only due to the increase in Jsc. To understand the reason, J01

and J02 values were compared.

96

As shown in Figure 86 (a), J01 values are order of 10-12

A/cm2 which is the

normal range and the values almost same except for reference. The J01 of reference is

slightly higher than the others. Thus, reverse saturation current component is due to

the emitter and bulk recombination.

15 20 25 30 35 40 451.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

70UA

60UA

Reference

J0

1 (

pA

/ c

m2)


(a)

15 20 25 30 35 40 450

50

100

150

200

250

300

J0

2 (

nA

/ c

m2)


70UA

60UA

Reference

(b)

Figure 86: a) J01 b) J02 with respect to texturing time

Figure 86 (b) shows J02 values which are order of 10-10

A/cm2

for wafers

60UA 25min and 35min, 70UA 15min, and reference. For the others, they are in

order of 10-9

A/cm2. The values of J02 higher than 10

-10 A/cm

2 shows possible

junction related problems [42].

We can than conclude that low Voc values for the reference sample is

possibly due to the higher recombination of electron hole pairs at the front surface

and low junction quality that leads to higher reverse saturation current.

To observe whether there are high series resistance due to contacts or not,

electroluminescence images like shown in Figure 87 of produced cells were taken.

The all images show almost same characteristics, so the others are not added here.

All images have dots which are due to the contact point of transporters in firing

furnace. There is only dead space at the edges of cells which are cut during edge

isolation.

97

(a)

(b)

Figure 87: Electroluminescence images of a) Reference b) 70UA 45min solar cell

Finally, external quantum efficiencies (EQEs) were measured to observe the

response of cells to different wavelengths. EQE is proportional to passivation and

reflection for short wavelengths, and it is proportional to light trapping for long

wavelengths in general.

Figure 88 a) and b) show the EQE and reflection results of cells with highest

efficiencies and reference. For 45 min texturing process at 60oC, the EQE is higher

than that of reference cell between 350 nm and 700 nm, while reflection is lower

than that of reference. Although the reflection value is slightly higher than that of

reference beyond 700 nm, the EQE is higher than that of reference between 850 nm

and 1050 nm. For 45 min texturing process at 70oC, the situation is similar. Between

350nm and 700nm, the reflection value of 70UA_45min cell is lower than reference;

EQE is higher than that of reference. And EQE is higher than that of reference

between 750 nm and 1100 nm, as the reflection is also higher than that of reference.

These results show that the blue response and the light trapping are higher than those

of reference cell.

98

400 500 600 700 800 900 1000 1100

0

10

20

30

40

50

60

70

80

90

100

E

QE

an

d R

efl

ec

tio

n (

%)

Wavelength (nm)

Reference_EQE

Reference_Reflection

60UA_45min_EQE

60UA_45min_Reflection

(a)

400 500 600 700 800 900 1000 1100

0

10

20

30

40

50

60

70

80

90

100

E.Q

.E a

nd

Re

fle

cti

on

(%

)

Wavelength (nm)

Reference_EQE

Reference_Reflection

70UA_45min_EQE

70UA_45min_Reflection

(b)

400 500 600 700 800 900 1000 1100

30

40

50

60

70

80

90

100

110

I.Q

.E (

%)

Wavelength (nm)

Reference_IQE

60UA_45min_IQE

(c)

400 500 600 700 800 900 1000 1100

30

40

50

60

70

80

90

100

I.Q

.E (

%)

Wavelength (nm)

Reference_IQE

70UA_45min_IQE

(d)

Figure 88: EQE and Reflection-IQE of solar cells produced by using wafers textured

for 45 min a)-c) at 60oC and b)-d) at 70

oC

To observe whether the blue response is higher due to only the reflection

value or not, internal quantum efficiency -wavelength (IQE) curves are also added as

shown in Figure 88 c) and d). Because IQE shows the performance of cells by

excluding the reflection losses.

IQE values are higher than that of reference as shown in Figure 88. It can be

said that the passivation quality of wafers with ultrasonic texturing is higher than that

of reference. To understand the reason, the SEM images of reference wafer after

texturing process was taken. Because, although all process steps are optimized for

the reference, the produced cells with wafer that were textured with ultrasonic

agitation shows better performance than the reference cells. If the quality of process

99

steps, junction quality and wafer quality (see Figure 86 for junction quality and bulk

quality) are same for all samples, the only difference will be the surface

morphologies.

Figure 89 shows the SEM images of wafers whose EQE, IQE and reflection

values are given in Figure 88. As can be seen, the surface morphologies of wafers

textured with ultrasonic agitation are more homogenous than the reference cell. The

reference cell has more small pyramids on big pyramids and big pyramids formed by

small pyramids. This inhomogenity of the surface makes difficult to get good

passivation quality which is related to the front surface recombination of the cells.

Figure 89: SEM image of a) 70UA 45min b) 60UA 45 min c) reference wafer after

texturing process

101

CHAPTER 6

CONCULUSIONS

Texturing process is the first step of solar cell production. This process is

carried out by using KOH-IPA solution to minimize the optical losses by forming

random pyramids on wafer surface and thus increases the efficiency of the solar cell.

In this study, the process parameters like temperature, time, and concentrations of

KOH & IPA were optimized by using magnetic agitation. Then process temperature

and duration were minimized by using ultrasonic agitation. Major conclusions of our

studies are summarized below.

The results related with magnetic agitation are;

1. As the process duration increases, covered area and pyramid sizes increases for

magnetic agitation. But it is not enough to get fully covered surfaces by just

increasing the time. All process parameters must be optimized.

2. For process parameters 4.2 wt% KOH, 5.4vol% IPA, as-cut and SDE wafers had

lowest reflection values for 45 min process duration at 70, 75 and 80oC. But

when the reflection results for 45 min were compared, lowest value was obtained

at 75oC. The difference between the highest and lowest reflection was 8% and

6% for as-cut and SDE respectively for 45 min process time.

3. For process parameters 3.3 wt% KOH, 5.4vol% IPA, again the lowest reflection

values were obtained for 45 min process time. The difference between the

highest and lowest reflection was 1% and 2% for as-cut and SDE respectively

for 45 min process time. It shows that these process parameters were less

sensitive to temperature when compared with previous one. Wafers textured

with 3.3wt% KOH had lower reflection values than wafers textured with 4.2wt

% KOH for 15, 25, and 35 min texturing time. This shows that decreasing KOH

102

concentration from 4.2 wt% to 3.3 wt% improved the process for short process

times.

4. A cap was used to close the glass beaker in which processes were carried out.

Closing the beaker increases the pressure on the solution and the evaporation of

water and IPA decreases. This makes the concentrations almost constant. For

process parameters 4.2 wt% KOH, 5.4 vol% IPA at 75oC, the wafers textured

for 15 min and 25 min had lower reflection than before. When the process was

carried out for 45 min, the pyramid densities increased for both as-cut and SDE

and this decreased the reflection slightly.

5. Increasing speed and changing IPA concentration did not improve process for

15 min and 25 min very much.

6. There are some problems faced during texturing process with magnetic

agitation.

The reproducibility of the process was hard for some cases in which the

process duration was lower or equal 25 min for all process temperature

especially 70oC.

The surfaces of wafers were not uniformly textured, and there were differences

in reflection between the faces again for process durations lower or equal to 25

min.

These results show that it is hard to minimize process parameters especially

time and temperature by using magnetic agitation. We decided to change agitation

type with ultrasonic agitation.

1. As time increases, reflection decreases again.

2. The wafer surface was fully covered by texturing with 3.3wt% KOH, 5.4 wt%

IPA for 15 min at 70oC and 35 min 60

oC.

3. 15 min texturing had higher reflection due to pyramids with round tips.

4. The pyramid density of ultrasonic agitation was higher than that of magnetic

one about 5 times. Therefore small pyramids were obtained.

5. Having low boiling point (82.4oC), which makes it necessary to re-dose IPA

into the solution due to evaporation, is the main problem of the standard

103

solution and causes consuming high concentrations of IPA and leads to high

production costs, because IPA is expensive. Therefore by using ultrasonic

agitation, cost effective process was obtained by decreasing temperature and

shorting process time.

We did also simulations of SDE, one side and two side uniformly textured wafers.

The results are;

1. The reflection for SDE wafer is not only due to light reflected from front

surface but also back surface for wavelength higher than 700 nm and thickness

lower than 70 m.

2. The absorption decreases for wavelength higher than 850 nm as thickness

decreases for simulation model one side and two side textured wafers whose

thicknesses higher or equal to 100 m.

3. One side textured wafers shows higher absorption than two side textured,

because the total reflection probability of light not coming perpendicular to flat

surface is higher than that of light to surface with pyramids.

The solar cell results of wafers textured with ultrasonic agitation are;

1. The Jsc value was higher than the reference cell (34.4 mA/ cm2)

about 1.25 mA/

cm2 for wafers with efficiency about 17.6%

2. The Voc values were higher than the reference cell (619 mV) about 4 mV for

almost all wafers textured with ultrasonic agitation.

3. The efficiencies were higher than the reference cell (17%) about at least 0.2%

and at most 0.7%. The highest efficiency was 17.74% of wafers textured for 45

min 60oC with ultrasonic agitation.

4. From EQE and IQE results, the wafers textured with ultrasonic agitation had

uniformly distributed pyramids with heights less than 4 µm leading had better

blue response, surface passivation and light trapping.

To sum up, the texturing process with ultrasonic agitation has a potential to

minimize the process parameters. If more experiments are made about this texturing

technique, more cost effective processes will be able to obtain. If the solar cell

104

production parameters are optimize for wafers textured with ultrasonic agitation,

higher efficiency values will be obtained.

105

BIBLIOGRAPHY

[1] “Enviromental & Regulatory Specialists.” [Online]. Available:

http://earsi.com/ggp01.htm. [Accessed: 12-Oct-2014].

[2] “ISE Photovoltaics Report.” [Online]. Available:

https://www.ise.fraunhofer.de/en/downloads-englisch/pdf-files-

englisch/photovoltaics-report-slides.pdf. [Accessed: 10-Nov-2014].

[3] “PVCDROM.” [Online]. Available: http://pveducation.org/pvcdrom/.

[Accessed: 12-Nov-2014].

[4] D. M. Chapin, C. S. Fuller, and G. L. Pearson, “A new silicon p-n junction

photocell for converting solar radiation into electrical power,” J. Appl. Phys.,

vol. 25, pp. 676–677, 1954.

[5] W. Schockley and H. J. Quesisser, “Detailed balance limit of efficiency of pn

junction solar cells,” J. Appl. Phys., vol. 32, pp. 510–519, 1951.

[6] J. Nestor and X. Quiebras, “Wet chemical textures for crystalline silicon solar

cells,” pp. 1–124, 2013.

[7] M. A. Green, “Silicon Solar Cells: State of Art,” Philosophical Transactions

of The Royal Society A, 2013. [Online]. Available:

http://dx.doi.org/10.1098/rsta.2011.0413. [Accessed: 12-Nov-2014].

[8] “Nanosolar CIGS Solar Cells.” [Online]. Available:

http://www.solarfeeds.com/rip-nanosolar/. [Accessed: 01-Jan-2015].

[9] M.S. Thesis,E. H. Çiftpınar, Selective Emitter Formation via Single Step

Doping Through Laser Patterned Mask Oxide Layer for Monocrystalline

Silicon Cells: Middle East Technical University, 2014.

[10] M.S. Thesis, S. H. Sedani, Fabrication and Doping of Thin Crystalline Si

Prepared by E-Beam Evaporation on Glass Substrate: Middle East Technical

University, 2013.

[11] B. M. Kayes, L. Zhang, R. Twist, I. Ding, and G. S. Higashi, “Flexible Thin-

Film Tandem Solar Cells With > 30 % Efficiency,” IEEE J. Photovoltaics,

vol. 4, no. 2, pp. 729–733, 2014.

106

[12] M.S. Thesis, F. Es, Fabrication and Characterization of Single Crystalline

Silicon Solar Cells: Middle East Technical University, 2010.

[13] G. M. Research, “Building Integrated Photovoltaics: An emerging market.”

[Online]. Available: http://www.solarserver.com/solar-magazine/solar-

report/solar-report/building-integrated-photovoltaics-an-emerging-

market.html. [Accessed: 15-Dec-2014].

[14] “NSF / ONR Workshop on Key Scientific and Technological Issues for

Development of Next-Generation Organic Solar Cells.” [Online]. Available:

http://web.utk.edu/~opvwshop/. [Accessed: 01-Jan-2015].

[15] C. S. Solanki, “Solar Photovoltaics: Fundamentals Technologies And

Applications,” PHI Learning Pvt. Ltd., 2009, pp. 323–324.

[16] B. Hoffman, V. Sivakov, S. W. Schmitt, M. Y. Bashouti, M. Latzel, J. Dluhoš,

and J. Jiruse, “Wet – Chemically Etched Silicon Nanowire Solar Cells:

Fabrication and Advanced Characterization,” P. X. Peng, Ed. 2012, pp. 211–

230.

[17] “Series Resistance.” [Online]. Available:

http://pveducation.org/pvcdrom/solar-cell-operation/series-resistance.

[Accessed: 01-Feb-2015].

[18] “Quantum Efficiency.” [Online]. Available:

http://www.pveducation.org/pvcdrom/solar-cell-operation/quantum-efficiency.

[Accessed: 01-Feb-2015].

[19] J. D. Plummer, M. D. Deal, and P. B. Griffin, “Silicon VLSI technology:

fundamentals, practice and modeling,” Prentice Hall, 2000, pp. 618–619.

[20] R. N. Ghoshtagore, “Phosphorus diffusion processes in SiO2 films,” Thin

Solid Films, vol. 25, no. 2, pp. 501–513, 1975.

[21] K. Sakamoto, K. Nishi, F. Ichikawa, and S. Ushio, “Segregation and Transport

Coefficients of Impurities at the Si/SiO2 Interface,” J. Appl. Phys., vol. 61, no.

4, pp. 1553–1555, 1987.

[22] M.S. Thesis, M. Bilgen, Comparison of Crystal Si Solar Cells Fabricated on

P-Type and N-Type Float Zone Silicon Wafers: Middle East Technical

University, 2013.

[23] J. Nelson, The Physics of Solar Cells, 1st ed. London: Imperial College Press,

2003.

107

[24] “Anti-Reflection Coatings.” [Online]. Available:

http://pveducation.org/pvcdrom/design/anti-reflection-coatings. [Accessed:

01-May-2015].

[25] B. Streetman and S. Banerjee, Solid State Electronic Devices, 6th ed. PHI

Learning, 2009.

[26] F. Colville and C. Dunsky, “Lasers Support Emerging Solar Industry Needs,”

Photonic Spectra, 2008. [Online]. Available:

http://www.photonics.com/Article.aspx?AID=33002. [Accessed: 01-Feb-

2015].

[27] “Reference Solar Spectral Irradiance: ASTM G-173.” [Online]. Available:

http://rredc.nrel.gov/solar/spectra/am1.5/astmg173/astmg173.html. [Accessed:

23-Jun-2015].

[28] M. Moreno, D. Murias, J. Martínez, C. Reyes-Betanzo, a. Torres, R.

Ambrosio, P. Rosales, P. Roca i Cabarrocas, and M. Escobar, “A comparative

study of wet and dry texturing processes of c-Si wafers for the fabrication of

solar cells,” Sol. Energy, vol. 101, pp. 182–191, 2014.

[29] H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgärtel, “Anisotropic

etching of crystalline silicon in alkaline solutions,” J. Electrochem. Soc., vol.

137, no. 11, p. 3612, 1990.

[30] D. B. Lee, “Anisotropic etching of silicon,” J. Appl. Phys., vol. 40, no. 11, pp.

4569–4574, 1969.

[31] E. D. Palik, “A raman study of etching silicon in aqueous KOH,” J.

Electrochem. Soc., vol. 130, no. 4, p. 956, 1983.

[32] J. B. Price, “Anisotropic etching of silicon with KOH-H2O-isopropyl

alcohol,” J. Electrochem. Soc., vol. 120, no. 3, p. C96, 1973.

[33] I. Kashkoush, G. Chen, and D. Nemeth, “Characterization of c-Si

Texturization in wet KOH/IPA and its Effect on Cell Efficiency,” pp. 5–8.

[34] A. K. Chu, J. S. Wang, Z. Y. Tsai, and C. K. Lee, “A simple and cost-effective

approach for fabricating pyramids on crystalline silicon wafers,” Sol. Energy

Mater. Sol. Cells, vol. 93, no. 8, pp. 1276–1280, 2009.

[35] L. Wang, F. Wang, X. Zhang, N. Wang, Y. Jiang, Q. Hao, and Y. Zhao,

“Improving efficiency of silicon heterojunction solar cells by surface texturing

of silicon wafers using tetramethylammonium hydroxide,” J. Power Sources,

vol. 268, pp. 619–624, 2014.

108

[36] K. Birmann, M. Demant, S. Rein, and B. Bläsi, “Optical Characterization of

Random Pyramid Texturization,” Small, no. September, pp. 5–9, 2011.

[37] J. D. Hylton, a. R. Burgers, and W. C. Sinke, “Alkaline Etching for

Reflectance Reduction in Multicrystalline Silicon Solar Cells,” J.

Electrochem. Soc., vol. 151, no. 6, p. G408, 2004.

[38] M.S. Thesis, O. Demircioğlu, Optmization of Metallization in Crystalline

Silicon Solar Cells: Middle East Technical University, 2012.

[39] “High Lambertian reflectance over their effective spectral range.” [Online].

Available: https://www.labsphere.com/wp-

content/uploads/2015/05/Spectralon-Diffuse-Reflectance-Standards.pdf.

[Accessed: 13-Dec-2014].

[40] R. A. Sinton, A. Cuevas, and M. Stuckings, “Quasi-steady-state

photoconductance, a new method for solar cell material device and

characterization,” in Proc. 25th Photovoltaic Specialists Conference, 1996, pp.

457–460.

[41] “Optical Properties of Silicon.” [Online]. Available:

http://pveducation.org/pvcdrom/materials/optical-properties-of-silicon.

[Accessed: 01-May-2015].

[42] O. Breitenstein, J. Bauer, P. P. Altermatt, and K. Ramspeck, “Influence of

Defects on Solar Cell Characteristics,” Solid State Phenom., vol. 156–158, pp.

1–10, 2009.

structuring of surface for light management in monocrystalline si solar

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